arch: arc: small optimization in mpu driver
arch: arc: small optimization in mpu driver

to avoid _get_num_regions to access the aux
reg each time in the for loop

Signed-off-by: Wayne Ren <d5c6d7c4aa45e53f819f17870783d153a32ba7d0@synopsys.com>

kraj/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,kraj/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,kraj/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr