smc: make SmcRegisters explicitly !Send and !Sync

The handle owns one hardware controller exclusively. Replace the
misleading `PhantomData<UnsafeCell<()>>` marker (commented "allow Send")
with `PhantomData<*const ()>` so the type is both !Send and !Sync,
preventing the handle from being moved across threads or into an ISR
where it could alias the controller it owns.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

target/ast10x0/peripherals/smc/registers.rs

diff --git a/target/ast10x0/peripherals/smc/registers.rs b/target/ast10x0/peripherals/smc/registers.rs
index f629249..084a2de 100644
--- a/target/ast10x0/peripherals/smc/registers.rs
+++ b/target/ast10x0/peripherals/smc/registers.rs
@@ -19,7 +19,6 @@
 //! The controller layer answers "what to do with the data based on the topology."
 
 use ast1060_pac as device;
-use core::cell::UnsafeCell;
 use core::marker::PhantomData;
 
 use crate::smc::helpers::{
@@ -35,7 +34,11 @@
 /// hex offsets (e.g., `fmc000()` for offset 0x00, `fmc080()` for offset 0x80).
 pub struct SmcRegisters {
     base: *const device::fmc::RegisterBlock,
-    _not_sync: PhantomData<UnsafeCell<()>>, // Prevent Sync, allow Send
+    // `*const ()` marker keeps the handle `!Send` and `!Sync`. An `SmcRegisters`
+    // represents exclusive ownership of one hardware controller; it must not be
+    // shared between threads or moved into another execution context (e.g. an
+    // ISR) where it could alias the controller it owns.
+    _not_send_sync: PhantomData<*const ()>,
 }
 
 impl SmcRegisters {
@@ -49,7 +52,7 @@
     pub const unsafe fn new(base: *const device::fmc::RegisterBlock) -> Self {
         Self {
             base,
-            _not_sync: PhantomData,
+            _not_send_sync: PhantomData,
         }
     }