arch: arc: implement MPU_GAP_FILLING for mpu version 3

when MPU_GAP_FILLING is configured, the default mpu entry
(kernel read + kernel write) will be used to fill the gaps
among mpu entires to avoid dynamic mpu region splitting.
This will bring better performance in thread switch but fewer
constraints on privileged codes.

when MPU_GAP_FILLING is not configured, a sw-based mpu dynamic
region splitting is used to bypass the limitation of no mpu region
overlap in hardware. This approach will consume more hardware
mpu entries and more time in thread switch.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>