virtio: Add platform bus driver for memory mapped virtio device

This patch, based on virtio PCI driver, adds support for memory
mapped (platform) virtio device. This should allow environments
like qemu to use virtio-based block & network devices even on
platforms without PCI support.

One can define and register a platform device which resources
will describe memory mapped control registers and "mailbox"
interrupt. Such device can be also instantiated using the Device
Tree node with compatible property equal "virtio,mmio".

Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Michael S.Tsirkin <mst@redhat.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/Kconfig

@@ -35,4 +35,15 @@ config VIRTIO_BALLOON
 
 	 If unsure, say M.
 
+ config VIRTIO_MMIO
+ 	tristate "Platform bus driver for memory mapped virtio devices (EXPERIMENTAL)"
+ 	depends on EXPERIMENTAL
+ 	select VIRTIO
+ 	select VIRTIO_RING
+ 	---help---
+ 	 This drivers provides support for memory mapped virtio
+	 platform device driver.
+
+ 	 If unsure, say N.
+
 endmenu