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>

Documentation/devicetree/bindings/virtio/mmio.txt

@@ -0,0 +1,17 @@
+* virtio memory mapped device
+
+See http://ozlabs.org/~rusty/virtio-spec/ for more details.
+
+Required properties:
+
+- compatible:	"virtio,mmio" compatibility string
+- reg:		control registers base address and size including configuration space
+- interrupts:	interrupt generated by the device
+
+Example:
+
+	virtio_block@3000 {
+		compatible = "virtio,mmio";
+		reg = <0x3000 0x100>;
+		interrupts = <41>;
+	}