arch: arm: introduce/implement API for configuring static MPU regions

This commit introduces the generic ARM (core) API, which allows
the user to program a set of static (fixed) MPU regions at boot
time. The API function is invoked upon initialization, in the
ARM-specific call of _arch_switch_to_main_thread(). The API
implementation is provided in arm_core_mpu.c.

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