arm: nxp: mpu: Fix off-by-1 error in region index calculation

Both the ARM and NXP MPU drivers incorrectly calculated the region index
by assuming the region type (e.g., THREAD_STACK_GUARD_REGION) was
zero-indexed, when in reality it is one-indexed. This had the effect of
wasting one region.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>