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/Kconfig b/drivers/virtio/Kconfig
index b72477a..753912f 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -29,6 +29,14 @@
 	  This driver provides support for memory mapped virtio
 	  platform device driver.
 
+config VIRTIO_PCI
+	bool "PCI driver for virtio devices"
+	depends on DM_PCI
+	select VIRTIO
+	help
+	  This driver provides support for virtio based paravirtual device
+	  drivers over PCI.
+
 config VIRTIO_NET
 	bool "virtio net driver"
 	depends on VIRTIO