rpmsg: add api for getting the underlying virtio device

In order to retrieve the remoteproc structure from a rpmsg channel, add
a function for retrieving the virtio device. The associated remoteproc
structure can then be deduced from it.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Suman Anna <s-anna@ti.com>