virtio: ring: Introduce dma mapping for real devices

Real hardware devices need to use the dma mapping functions to work.
Blindly calling sg_phys and expecting it to work on any address
is a fool's errand.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
virtio_console: Simplify virtio_console for h/w devices.

Now that virtio_ring properly maps the buffers there is no need
for special handling of rproc_serial, nor any need to use
dma_alloc_coherent for the buffers; the standard mapping works,
plus sg_phys() on a piece of dma_alloc_coherent memory is bogus.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>