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/drivers/Makefile b/drivers/Makefile
index fb38b67..4453c62 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -14,6 +14,7 @@
 obj-$(CONFIG_$(SPL_TPL_)SPI_FLASH_SUPPORT) += mtd/spi/
 obj-$(CONFIG_$(SPL_TPL_)SPI_SUPPORT) += spi/
 obj-$(CONFIG_$(SPL_TPL_)TIMER) += timer/
+obj-$(CONFIG_$(SPL_TPL_)VIRTIO) += virtio/
 obj-$(CONFIG_$(SPL_)DM_MAILBOX) += mailbox/
 obj-$(CONFIG_$(SPL_)REMOTEPROC) += remoteproc/