arch: arm: mpu: Fix -Wstringop-overread warning

GCC 12 performs bounds checking on the pointer arguments specified like
an array (e.g. `int arg[]`) and treats such arguments with an empty
length as having the length of 0, resulting in the compiler printing
out `stringop-overread' warning when they are accessed.

This commit corrects any pointer arguments declared using the array
expression to use the pointer expression instead.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>