virtio: Add virtio over mmio transport driver

VirtIO can use various different buses and virtio devices are
commonly implemented as PCI devices. But virtual environments
without PCI support (a common situation in embedded devices
models) might use simple memory mapped device (“virtio-mmio”)
instead of the PCI device.

This adds a transport driver that implements UCLASS_VIRTIO for
virtio over mmio.

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/Kconfig b/drivers/virtio/Kconfig
index 82fc536..4f9a11b 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -22,4 +22,11 @@
 	  This option is selected by any driver which implements the virtio
 	  transport, such as CONFIG_VIRTIO_MMIO or CONFIG_VIRTIO_PCI.
 
+config VIRTIO_MMIO
+	bool "Platform bus driver for memory mapped virtio devices"
+	select VIRTIO
+	help
+	  This driver provides support for memory mapped virtio
+	  platform device driver.
+
 endmenu