virtio_ring: add virtqueue_add_inbuf/outbuf_rpmsg API

Expose new variants of virtqueue_add_inbuf() & virtqueue_add_outbuf()
API specifically to deal with virtio_rpmsg. The virtio core in general
expects all the vring buffers to be allocated from linear addresses,
but the virtio_rpmsg can have the buffers in non-linear space due to
its usage of the dma_alloc_coherent() API.

Based on a RFC patch from Edgar E. Iglesias <edgar.iglesias@xilinx.com>,
http://marc.info/?l=linux-virtualization&m=143047902512226&w=2



Signed-off-by: Suman Anna <s-anna@ti.com>