remoteproc: avoid registering a virtio device if not supported

Let remoteproc know when the firmware doesn't support any virtio
functionality, so registering a virtio device can be avoided.

This is needed for remote processors that doesn't require any
virtio-based communications, but are still controlled via remoteproc.

[ohad@wizery.com: write commit log]

Signed-off-by: Mark Grosen <mgrosen@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>