Documentation: dma: Add documentation for the APM X-Gene SoC DMA device DTS binding

This patch adds documentation for the APM X-Gene SoC DMA device DTS binding
Signed-off-by: Rameshwar Prasad Sahu <rsahu@apm.com>
Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/apm-xgene-dma.txt

+Applied Micro X-Gene SoC DMA nodes
+
+DMA nodes are defined to describe on-chip DMA interfaces in
+APM X-Gene SoC.
+
+Required properties for DMA interfaces:
+- compatible: Should be "apm,xgene-dma".
+- device_type: set to "dma".
+- reg: Address and length of the register set for the device.
+  It contains the information of registers in the following order:
+  1st - DMA control and status register address space.
+  2nd - Descriptor ring control and status register address space.
+  3rd - Descriptor ring command register address space.
+  4th - Soc efuse register address space.
+- interrupts: DMA has 5 interrupts sources. 1st interrupt is
+  DMA error reporting interrupt. 2nd, 3rd, 4th and 5th interrupts
+  are completion interrupts for each DMA channels.
+- clocks: Reference to the clock entry.
+
+Optional properties:
+- dma-coherent : Present if dma operations are coherent
+
+Example:
+	dmaclk: dmaclk@1f27c000 {
+		compatible = "apm,xgene-device-clock";
+		#clock-cells = <1>;
+		clocks = <&socplldiv2 0>;
+		reg = <0x0 0x1f27c000 0x0 0x1000>;
+		reg-names = "csr-reg";
+		clock-output-names = "dmaclk";
+	};
+
+	dma: dma@1f270000 {
+			compatible = "apm,xgene-storm-dma";
+			device_type = "dma";
+			reg = <0x0 0x1f270000 0x0 0x10000>,
+			      <0x0 0x1f200000 0x0 0x10000>,
+			      <0x0 0x1b008000 0x0 0x2000>,
+			      <0x0 0x1054a000 0x0 0x100>;
+			interrupts = <0x0 0x82 0x4>,
+				     <0x0 0xb8 0x4>,
+				     <0x0 0xb9 0x4>,
+				     <0x0 0xba 0x4>,
+				     <0x0 0xbb 0x4>;
+			dma-coherent;
+			clocks = <&dmaclk 0>;
+	};