documentation: iommu: Add binding document of Exynos System MMU

This patch adds a description of the device tree binding for the
Samsung Exynos System MMU.
Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>
Signed-off-by: Shaik Ameer Basha <shaik.ameer@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/devicetree/bindings/iommu/samsung,sysmmu.txt

+Samsung Exynos IOMMU H/W, System MMU (System Memory Management Unit)
+
+Samsung's Exynos architecture contains System MMUs that enables scattered
+physical memory chunks visible as a contiguous region to DMA-capable peripheral
+devices like MFC, FIMC, FIMD, GScaler, FIMC-IS and so forth.
+
+System MMU is an IOMMU and supports identical translation table format to
+ARMv7 translation tables with minimum set of page properties including access
+permissions, shareability and security protection. In addition, System MMU has
+another capabilities like L2 TLB or block-fetch buffers to minimize translation
+latency.
+
+System MMUs are in many to one relation with peripheral devices, i.e. single
+peripheral device might have multiple System MMUs (usually one for each bus
+master), but one System MMU can handle transactions from only one peripheral
+device. The relation between a System MMU and the peripheral device needs to be
+defined in device node of the peripheral device.
+
+MFC in all Exynos SoCs and FIMD, M2M Scalers and G2D in Exynos5420 has 2 System
+MMUs.
+* MFC has one System MMU on its left and right bus.
+* FIMD in Exynos5420 has one System MMU for window 0 and 4, the other system MMU
+  for window 1, 2 and 3.
+* M2M Scalers and G2D in Exynos5420 has one System MMU on the read channel and
+  the other System MMU on the write channel.
+The drivers must consider how to handle those System MMUs. One of the idea is
+to implement child devices or sub-devices which are the client devices of the
+System MMU.
+
+Required properties:
+- compatible: Should be "samsung,exynos-sysmmu"
+- reg: A tuple of base address and size of System MMU registers.
+- interrupt-parent: The phandle of the interrupt controller of System MMU
+- interrupts: An interrupt specifier for interrupt signal of System MMU,
+	      according to the format defined by a particular interrupt
+	      controller.
+- clock-names: Should be "sysmmu" if the System MMU is needed to gate its clock.
+	       Optional "master" if the clock to the System MMU is gated by
+	       another gate clock other than "sysmmu".
+	       Exynos4 SoCs, there needs no "master" clock.
+	       Exynos5 SoCs, some System MMUs must have "master" clocks.
+- clocks: Required if the System MMU is needed to gate its clock.
+- samsung,power-domain: Required if the System MMU is needed to gate its power.
+	  Please refer to the following document:
+	  Documentation/devicetree/bindings/arm/exynos/power_domain.txt
+
+Examples:
+	gsc_0: gsc@13e00000 {
+		compatible = "samsung,exynos5-gsc";
+		reg = <0x13e00000 0x1000>;
+		interrupts = <0 85 0>;
+		samsung,power-domain = <&pd_gsc>;
+		clocks = <&clock CLK_GSCL0>;
+		clock-names = "gscl";
+	};
+
+	sysmmu_gsc0: sysmmu@13E80000 {
+		compatible = "samsung,exynos-sysmmu";
+		reg = <0x13E80000 0x1000>;
+		interrupt-parent = <&combiner>;
+		interrupts = <2 0>;
+		clock-names = "sysmmu", "master";
+		clocks = <&clock CLK_SMMU_GSCL0>, <&clock CLK_GSCL0>;
+		samsung,power-domain = <&pd_gsc>;
+	};