rpmsg virtio: add rpmsg virtio wrapper support

Rpmsg VirtIO is a virtio transport implementation for Rpmsg, and
it's different to the rptun framework.

rpmsg_virtio.c implements the rpmsg virtio transport layer by itself
to avoid use the remoteproc implementation in OpenAMP to save code
size, so it can be treated as a lightweight version of rptun.
Therefore, rpmsg_virtio.c only support the communication feature and
do not support contoll the life cycle of the remote core.
But benefit by it's small footprint, it can be used in the chips with
small flash.

Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

drivers/rpmsg/Kconfig

@@ -22,3 +22,20 @@ config RPMSG_PING
 		channel, user can use it to get send/recv speed & latency.
 
 endif # RPMSG
+
+config RPMSG_VIRTIO
+	bool "rpmsg virtio transport support"
+	default n
+	select RPMSG
+
+if RPMSG_VIRTIO
+
+config RPMSG_VIRTIO_PRIORITY
+	int "rpmsg virtio rx thread priority"
+	default 224
+
+config RPMSG_VIRTIO_STACKSIZE
+	int "rpmsg virtio rx thread stack size"
+	default DEFAULT_TASK_STACKSIZE
+
+endif