Documentation: dma: Add documentation for ZTE DMA

This patch adds documentation for the ZTE ZX296702 SoC DMA device
DTS binding.
Signed-off-by: NJun Nie <jun.nie@linaro.org>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/zxdma.txt

+* ZTE ZX296702 DMA controller
+
+Required properties:
+- compatible: Should be "zte,zx296702-dma"
+- reg: Should contain DMA registers location and length.
+- interrupts: Should contain one interrupt shared by all channel
+- #dma-cells: see dma.txt, should be 1, para number
+- dma-channels: physical channels supported
+- dma-requests: virtual channels supported, each virtual channel
+		have specific request line
+- clocks: clock required
+
+Example:
+
+Controller:
+	dma: dma-controller@0x09c00000{
+		compatible = "zte,zx296702-dma";
+		reg = <0x09c00000 0x1000>;
+		clocks = <&topclk ZX296702_DMA_ACLK>;
+		interrupts = <GIC_SPI 66 IRQ_TYPE_LEVEL_HIGH>;
+		#dma-cells = <1>;
+		dma-channels = <24>;
+		dma-requests = <24>;
+	};
+
+Client:
+Use specific request line passing from dmax
+For example, spdif0 tx channel request line is 4
+	spdif0: spdif0@0b004000 {
+		#sound-dai-cells = <0>;
+		compatible = "zte,zx296702-spdif";
+		reg = <0x0b004000 0x1000>;
+		clocks = <&lsp0clk ZX296702_SPDIF0_DIV>;
+		clock-names = "tx";
+		interrupts = <GIC_SPI 21 IRQ_TYPE_LEVEL_HIGH>;
+		dmas = <&dma 4>;
+		dma-names = "tx";
+	}