virtio: Add virtio over pci transport driver

This adds a transport driver that implements UCLASS_VIRTIO for
virtio over pci, which is commonly used on x86.

It only supports the legacy interface of the pci transport, which
is the default device that QEMU emulates.

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 5fe7428..5ee6183 100644
--- a/drivers/virtio/Makefile
+++ b/drivers/virtio/Makefile
@@ -5,5 +5,6 @@
 
 obj-y += virtio-uclass.o virtio_ring.o
 obj-$(CONFIG_VIRTIO_MMIO) += virtio_mmio.o
+obj-$(CONFIG_VIRTIO_PCI) += virtio_pci.o
 obj-$(CONFIG_VIRTIO_NET) += virtio_net.o
 obj-$(CONFIG_VIRTIO_BLK) += virtio_blk.o