* 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.

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