dt-bindings: iommu: Add documentation for rockchip iommu

Rockchip IOMMU

==============

A Rockchip DRM iommu translates io virtual addresses to physical addresses for

its master device.  Each slave device is bound to a single master device, and

shares its clocks, power domain and irq.

Required properties:

- compatible      : Should be "rockchip,iommu"

- reg             : Address space for the configuration registers

- interrupts      : Interrupt specifier for the IOMMU instance

- interrupt-names : Interrupt name for the IOMMU instance

- #iommu-cells    : Should be <0>.  This indicates the iommu is a

                    "single-master" device, and needs no additional information

                    to associate with its master device.  See:

                    Documentation/devicetree/bindings/iommu/iommu.txt

Example:

	vopl_mmu: iommu@ff940300 {

		compatible = "rockchip,iommu";

		reg = <0xff940300 0x100>;

		interrupts = <GIC_SPI 16 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-names = "vopl_mmu";

		#iommu-cells = <0>;

	};