arch: arm: implement internal function to disable MPU region

This commit adds an internal function to disable an MPU region.
The function includes an assert that the requested MPU region
number is a valid one. arm_mpu.c is refactor to use this
function in all cases where an MPU region needs to be disabled.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>