Fix MPU region enablement in stack guard installation

Updated the MPU region enablement logic in the `install_stack_guard` function to correctly set the region limit by using the stack bottom address plus 256 minus one, ensuring proper memory protection configuration.

See Table 235. MPU_RLAR Register in RP2350 documentation
See Section 4.5 MPU_RLAR in armv8m MPU documentation

Author: adamNewell

embassy-rp/src/lib.rs

@@ -567,7 +567,7 @@ unsafe fn install_stack_guard(stack_bottom: *mut usize) -> Result<(), ()> {
     unsafe {
         core.MPU.ctrl.write(5); // enable mpu with background default map
         core.MPU.rbar.write(stack_bottom as u32 & !0xff); // set address
-        core.MPU.rlar.write(1); // enable region
+        core.MPU.rlar.write(((stack_bottom as usize + 255) as u32) | 1);
     }
     Ok(())
 }