remoteproc: don't use virtio's weak barriers

When creating a virtqueue for rpmsg, tell virtio we're not interested
in "weak" smp barriers, since we're talking to a real device.

On ARM, this means using a DSB instead of a DMB, which is needed
for platforms that kick the remote processor using some kind of
a mailbox device mapped to Device memory (otherwise the kick can
jump ahead and wake the remote processor before it has observed
the changes to the vrings).

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>