dm: Add a new uclass driver for VirtIO transport devices

This adds a new virtio uclass driver for “virtio” [1] family of
devices that are are found in virtual environments like QEMU,
yet by design they look like physical devices to the guest.

The uclass driver provides child_pre_probe() and child_post_probe()
methods to do some common operations for virtio device drivers like
device and driver supported feature negotiation, etc.

[1] http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.pdf

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/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 269a2c6..c91dca1 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -96,6 +96,7 @@
 	UCLASS_VIDEO_BRIDGE,	/* Video bridge, e.g. DisplayPort to LVDS */
 	UCLASS_VIDEO_CONSOLE,	/* Text console driver for video device */
 	UCLASS_VIDEO_OSD,	/* On-screen display */
+	UCLASS_VIRTIO,		/* VirtIO transport device */
 	UCLASS_W1,		/* Dallas 1-Wire bus */
 	UCLASS_W1_EEPROM,	/* one-wire EEPROMs */
 	UCLASS_WDT,		/* Watchdot Timer driver */