rpmsg: fix kconfig dependencies for VIRTIO

Fix this:
warning: (VIRTIO_PCI && VIRTIO_MMIO && REMOTEPROC && RPMSG) selects VIRTIO which has unmet direct dependencies (VIRTUALIZATION)

Cc: stable@vger.kernel.org
Signed-off-by: Suman Anna <s-anna@ti.com>
[edit commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>