arm: nxp: mpu: Return constant number of mpu regions

The original implementation of _get_num_regions() parsed the CESR[NRGD]
register field to determine the number of mpu region descriptors
implemented in hardware. There was a possible path in the code to return
zero, which would cause underflow later on in arm_core_mpu_configure().
Coverity complained despite an assert to catch this condition. Instead,
use a preprocessor macro from mcux that defines the number of mpu region
descriptors.

Coverity-CID: 169811
Jira: ZEP-2208

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