target/ast10x0/peripherals: align PhantomData to *const () across all structs

Replace PhantomData<UnsafeCell<()>> (_not_sync) with
PhantomData<*const ()> (_not_send_sync) in all peripheral register
wrapper structs, and add the field where it was missing entirely.

- scu/registers.rs: rename _not_sync -> _not_send_sync, update type
- smc/registers.rs: rename _not_sync -> _not_send_sync, update type,
  remove now-unused UnsafeCell import
- i2c/controller.rs: rename _not_sync -> _not_send_sync, update type,
  remove now-unused UnsafeCell import
- sgpiom/register_block.rs: add missing _not_send_sync field and import
- uart/mod.rs: add missing _not_send_sync field and import

PhantomData<*const ()> is the conventional way to opt out of both Send
and Sync regardless of other field types, and makes the intent explicit
at the struct definition.

target/ast10x0/peripherals/scu/registers.rs

diff --git a/target/ast10x0/peripherals/scu/registers.rs b/target/ast10x0/peripherals/scu/registers.rs
index 3444b72..ceb7314 100644
--- a/target/ast10x0/peripherals/scu/registers.rs
+++ b/target/ast10x0/peripherals/scu/registers.rs
@@ -4,7 +4,6 @@
 //! AST10x0 SCU low-level register access.
 
 use ast1060_pac as device;
-use core::cell::UnsafeCell;
 use core::marker::PhantomData;
 
 const SCU_UNLOCK_KEY: u32 = 0x1688_A8A8;
@@ -12,7 +11,12 @@
 /// Safe wrapper around the AST10x0 SCU register block.
 pub struct ScuRegisters {
     base: *const device::scu::RegisterBlock,
-    _not_sync: PhantomData<UnsafeCell<()>>, // Prevent Sync, allow Send.
+    /// Prevent `Send` and `Sync`.
+    ///
+    /// MMIO register blocks must not be transferred across threads or
+    /// shared by reference due to potential side effects and lack of
+    /// synchronization guarantees.
+    _not_send_sync: PhantomData<*const ()>,
 }
 
 impl ScuRegisters {
@@ -23,10 +27,7 @@
     /// - `base` points to a valid SCU register block.
     /// - access to the SCU instance is serialized appropriately.
     const unsafe fn new(base: *const device::scu::RegisterBlock) -> Self {
-        Self {
-            base,
-            _not_sync: PhantomData,
-        }
+        Self { base, _not_send_sync: PhantomData }
     }
 
     /// Create a register accessor for the global SCU instance.

target/ast10x0/peripherals/sgpiom/register_block.rs

diff --git a/target/ast10x0/peripherals/sgpiom/register_block.rs b/target/ast10x0/peripherals/sgpiom/register_block.rs
index 35fa4ec..f227c42 100644
--- a/target/ast10x0/peripherals/sgpiom/register_block.rs
+++ b/target/ast10x0/peripherals/sgpiom/register_block.rs
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use ast1060_pac as device;
+use core::marker::PhantomData;
 
 use super::types::{
     Bank, BankDevice, Direction, Error, InitialLevel, InterruptMode, InterruptTrigger,
@@ -10,6 +11,12 @@
 
 pub struct Sgpiom {
     sgpiom: *const device::sgpiom::RegisterBlock,
+    /// Prevent `Send` and `Sync`.
+    ///
+    /// MMIO register blocks must not be transferred across threads or
+    /// shared by reference due to potential side effects and lack of
+    /// synchronization guarantees.
+    _not_send_sync: PhantomData<*const ()>,
 }
 
 impl Sgpiom {
@@ -21,7 +28,7 @@
     /// - The pointed register block must remain valid for the lifetime of this `Sgpiom`.
     /// - Caller must enforce global ownership so concurrent mutable access does not occur.
     pub const unsafe fn new(sgpiom: *const device::sgpiom::RegisterBlock) -> Self {
-        Self { sgpiom }
+        Self { sgpiom, _not_send_sync: PhantomData }
     }
 
     /// Create an instance pointing to the global AST1060 SGPIOM register block.

target/ast10x0/peripherals/uart/mod.rs

diff --git a/target/ast10x0/peripherals/uart/mod.rs b/target/ast10x0/peripherals/uart/mod.rs
index 7a07df4..ad45b7f 100644
--- a/target/ast10x0/peripherals/uart/mod.rs
+++ b/target/ast10x0/peripherals/uart/mod.rs
@@ -3,6 +3,7 @@
 
 use ast1060_pac as device;
 use bitflags::bitflags;
+use core::marker::PhantomData;
 use embedded_hal_nb::serial as serial_nb;
 use embedded_io::{Read, Write};
 
@@ -100,6 +101,12 @@
 }
 pub struct Usart {
     usart: *const device::uart::RegisterBlock,
+    /// Prevent `Send` and `Sync`.
+    ///
+    /// MMIO register blocks must not be transferred across threads or
+    /// shared by reference due to potential side effects and lack of
+    /// synchronization guarantees.
+    _not_send_sync: PhantomData<*const ()>,
 }
 
 impl embedded_io::ErrorType for Usart {
@@ -278,7 +285,7 @@
     /// - `usart` must be a valid, non-null pointer to the AST1060 UART register block.
     /// - The pointed register block must remain valid for the lifetime of this `Usart`.
     pub const unsafe fn new_uninit(usart: *const device::uart::RegisterBlock) -> Self {
-        Self { usart }
+        Self { usart, _not_send_sync: PhantomData }
     }
 
     /// Create a new USART instance from a raw register-block pointer.
@@ -293,7 +300,7 @@
     /// - Caller must enforce global ownership/coordination so concurrent mutable access
     ///   does not occur through other code paths.
     pub unsafe fn new(usart: *const device::uart::RegisterBlock) -> Self {
-        let this = Self { usart };
+        let this = Self { usart, _not_send_sync: PhantomData };
 
         unsafe {
             this.regs().uartfcr().write(|w| {