arch: arm: use CMSIS defines for MPU_RBAR register bit setting

This commit removes the macro definitions for MPU_RBAR register
bitmasks, defined in arm_mpu.h, and modifies the MPU driver to
directly use the equivalent macros defined in ARM CMSIS.

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