mctp: add WG object to mctp_server process (Phase 6 Step 1)

Add a wait_group object to the mctp_server process in the AST1060-EVB
MCTP system configuration.

The MCTP server needs to multiplex two event sources:
- user_data=0: IPC from a client (MCTP channel READABLE)
- user_data=1: I2C slave notification from the I2C server (I2C channel USER)

Without the WG object the kernel does not expose handle::WG to the
binary, so wait_group_add() calls in main.rs would have no valid handle.

target/ast1060-evb/mctp/system.json5

diff --git a/target/ast1060-evb/mctp/system.json5 b/target/ast1060-evb/mctp/system.json5
index d845ae4..510bf1a 100644
--- a/target/ast1060-evb/mctp/system.json5
+++ b/target/ast1060-evb/mctp/system.json5
@@ -72,6 +72,12 @@
                         handler_app: "i2c_server",
                         handler_object_name: "I2C",
                     },
+                    {
+                        // WaitGroup: multiplexes IPC channel (READABLE) and I2C slave
+                        // notification (USER). user_data=0 → IPC from a client, user_data=1 → inbound.
+                        name: "WG",
+                        type: "wait_group",
+                    },
                 ],
                 threads: [
                     {