arch: arm: mpu: replace literals with CMSIS bitsets

This commit refactors internal functions in arm_mpu.c to use
bitsets and functions taken directly from ARM CMSIS instead of
hardcoded arithmetic literals. In several internal functions
some part of the implementation is abstracted further in inline
functions or convenience macros, to facilitate extending the
arm_mpu.c for ARMv8-M. In addition, the commit adds minor
improvements in internal function documentation.

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