arch: arm: add additional sanity checks before MPU config change

This commit inserts sanity checks every time we are performing
a (re)-configuration of one or multiple MPU regions, ensuring
that we do not attempt to configure an invalid region number.

Particulary for arm_mpu_config(), called during pre-kernel
initialization phase, we add a system ASSERT if we attempt
to initialize more regions that what is supported by hardware.
We do this to ensure the misconfiguration is detected early and
the system boot is aborted.

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