virtio_ring: switch to dma_XX barriers for rpmsg

virtio is using barriers to order memory accesses, thus
dma_wmb/rmb is a good match.

Before
[mst@tuck linux]$ size drivers/virtio/virtio_ring.o
   text    data     bss     dec     hex filename
  11392     820       0   12212    2fb4 drivers/virtio/virtio_ring.o

After
mst@tuck linux]$ size drivers/virtio/virtio_ring.o
   text    data     bss     dec     hex filename
  11284     820       0   12104    2f48 drivers/virtio/virtio_ring.o

Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: linux-remoteproc@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>