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 <aurelien@aurel32.net>