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

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>;

	};