arch: arm: mpu: force outstanding transactions before MPU disabling
arch: arm: mpu: force outstanding transactions before MPU disabling

The ARMv8-M Memory Protection Unit document indicates that a DMB
instruction must be used before disabling the MPU in order to force
outstanding memory transactions.

The ARMv7-M documentation is less clear about that, and only specifies
that a DSB instruction followed by a ISB instruction must be used
before enabling the MPU, which is already the case. The ARMv7-M and
ARMv8-M MPU are relatively similar to believe the same sequence should
be used for disabling it.

This patch therefore adds a DMB instruction before disabling the MPU.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

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