rp: fix multicore stack guard setup

the region field of the register is four bits wide followed by the valid
bit that causes the rnr update we rely on for the rasr write. 0x08 is
just a bit short to reach the valid bit, and since rp2040 has only 8
regions it (at best) doesn't do anything at all.

embassy-rp/src/multicore.rs

@@ -74,7 +74,7 @@ fn install_stack_guard(stack_bottom: *mut usize) {
     let subregion_select = 0xff ^ (1 << ((addr >> 5) & 7));
     unsafe {
         core.MPU.ctrl.write(5); // enable mpu with background default map
-        core.MPU.rbar.write((addr & !0xff) | 0x8);
+        core.MPU.rbar.write((addr & !0xff) | (1 << 4)); // set address and update RNR
         core.MPU.rasr.write(
             1 // enable region
                | (0x7 << 1) // size 2^(7 + 1) = 256