arch: arm: introduce/implement API to configure dynamic MPU regions

This commit introduces an ARM API that allows the user to
program a set of dynamic MPU regions at run-time. The API
function is invoked every time the memory map needs to be
re-programmed (for example at thread context-switch). The
functionality is implementated in arm_core_mpu.c.

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