* virtio memory mapped device

See https://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

Required properties for virtio-iommu:

- #iommu-cells:	When the node corresponds to a virtio-iommu device, it is
		linked to DMA masters using the "iommus" or "iommu-map"
		properties [1][2]. #iommu-cells specifies the size of the
		"iommus" property. For virtio-iommu #iommu-cells must be
		1, each cell describing a single endpoint ID.

Optional properties:

- iommus:	If the device accesses memory through an IOMMU, it should
		have an "iommus" property [1]. Since virtio-iommu itself
		does not access memory through an IOMMU, the "virtio,mmio"
		node cannot have both an "#iommu-cells" and an "iommus"
		property.

- ring_base: 	Indicates the physical address of memory which needs to be
		used for hosting virtqueue ring structures of the device.
		Presence of this property also results in custom DMA
		ops being used for the device, which will use swiotlb APIs
		to bounce buffers.

Example:

	virtio_block@3000 {
		compatible = "virtio,mmio";
		reg = <0x3000 0x100>;
		interrupts = <41>;

		/* Device has endpoint ID 23 */
		iommus = <&viommu 23>
	}

	viommu: iommu@3100 {
		compatible = "virtio,mmio";
		reg = <0x3100 0x100>;
		interrupts = <42>;

		#iommu-cells = <1>
	}

[1] Documentation/devicetree/bindings/iommu/iommu.txt
[2] Documentation/devicetree/bindings/pci/pci-iommu.txt
