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>

arch/arm/core/cortex_m/mpu/arm_mpu.c

@@ -72,6 +72,9 @@ void arm_core_mpu_enable(void)
  */
 void arm_core_mpu_disable(void)
 {
+	/* Force any outstanding transfers to complete before disabling MPU */
+	__DMB();
+
 	/* Disable MPU */
 	MPU->CTRL = 0;
 }