arch: arm: mpu: improve documentation of mpu_config

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

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