A 'dma_wmb' barrier is enough to guarantee previous writes
before accessing by acc device in the outer shareable domain.

A 'smp_wmb' barrier is enough to guarantee previous writes
before accessing by other cpus in the inner shareble domain.
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>