virtio: Add codes for virtual queue/ring management

This adds support for managing virtual queue/ring, the channel
for high performance I/O between host and guest.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
index 23e7be7..17d264a 100644
--- a/drivers/virtio/Makefile
+++ b/drivers/virtio/Makefile
@@ -3,4 +3,4 @@
 # Copyright (C) 2018, Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
 # Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
 
-obj-y += virtio-uclass.o
+obj-y += virtio-uclass.o virtio_ring.o