dt-bindings: iommu: Add documentation for rockchip iommu

Add binding documentation for Rockchip IOMMU.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Simon Xue <xxm@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/devicetree/bindings/iommu/rockchip,iommu.txt

+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>;
+	};