arm: core: mpu: Add arm core MPU interface

This patch adds the arm core MPU interface, a common way to access the
pu functionalities by the arm zephyr kernel.

The interface can be divided in two parts:
- a core part that will be implemented by the arm_core_mpu driver and
  used directly by the kernel
- a driver part that will be implemented by the mpu drivers and used by
  the arm_core_mpu driver

Change-Id: I590bd284abc40d98b06fdf1efb5800903313aa00
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>