arch: arm: cortex_m: z_arm_mpu_init: fix D-Cache invalidation

In case CONFIG_NOCACHE_MEMORY=y, the D-Cache need to be clean and
invalidated before enabling the MPU to make sure no data from a
__nocache__ region is present in the D-Cache.

If the D-Cache is disabled, SCB_CleanInvalidateDCache() shall not be
used as it might contains random data for random addresses, and this
might just create a bus fault.

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