arch: arm: mpu: improve documentation of nxp_mpu_config

This commit enhances the documentation of the nxp_mpu_config
element in include/arch/arm/cortex_m/mpu/nxp_mpu.h, stressing
that it intends to store information for fixed MPU regions.

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