In arm_core_mpu_mem_partition_config_update() we must fix the
index value of the start of the for loop, to address the case
when dynamic MPU regions are programmed in index values lower
than static_regions_num. (This might occur in ARMv8-M MPU,
depending on the order of programming the map of the
dynamic regions in run-time). We introduce function
_get_dyn_region_min_index(), and provide the implementation in
arm_mpu_v8_internal.h and arm_mpu_v7_internal.h (trivial).
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>