arch: arm: mpu: fix erratum 1013783-B

Due to erratum 1013783-B, speculative accesses might be performed
to normal memory unmapped in the MPU. This can be avoided by using
MPU region 0 to cover all unmapped memory and make this region
execute-never and inaccessible.

Note that this applies when CPU Cortex-M7 implements MPU and D-cache.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>