arch: arm: mpu: get REGION_SIZE_<X> defines directly from ARM CMSIS

This commit re-defines the REGION_SIZE_<x> macros, present in
include/arch/arm/cortex_m/mpu/arm_mpu_v7m.h, with the help of
the existing ARM MPU regions size definitions in ARM CMSIS.

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