arm_mpu: fix off-by-one in mpu_buffer_validate

The loop that checks the MPU region permissions incorrectly
iterates from (num_region + 1) to 1. This should be
(num_region - 1) to 0. This results in the wrong regions
being checked and possible overflow for any implementation
that uses the max region value of 0xFF.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>