virtio: Add __virtqueue_add_sgs

Add __virtqueue_add_sgs with an option to use DMA mapped
scatterlists. This option is only meant to be used in combination
with remoteproc/rpmsg.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>