dt-bindings: dmaengine: Add Spreadtrum SC9860 DMA controller

This patch adds the binding documentation for Spreadtrum SC9860 DMA
controller device.

Signed-off-by: Baolin Wang <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/dma/sprd-dma.txt

···
+* Spreadtrum DMA controller
+
+This binding follows the generic DMA bindings defined in dma.txt.
+
+Required properties:
+- compatible: Should be "sprd,sc9860-dma".
+- reg: Should contain DMA registers location and length.
+- interrupts: Should contain one interrupt shared by all channel.
+- #dma-cells: must be <1>. Used to represent the number of integer
+	cells in the dmas property of client device.
+- #dma-channels : Number of DMA channels supported. Should be 32.
+- clock-names: Should contain the clock of the DMA controller.
+- clocks: Should contain a clock specifier for each entry in clock-names.
+
+Example:
+
+Controller:
+apdma: dma-controller@20100000 {
+	compatible = "sprd,sc9860-dma";
+	reg = <0x20100000 0x4000>;
+	interrupts = <GIC_SPI 50 IRQ_TYPE_LEVEL_HIGH>;
+	#dma-cells = <1>;
+	#dma-channels = <32>;
+	clock-names = "enable";
+	clocks = <&clk_ap_ahb_gates 5>;
+};
+
+
+Client:
+DMA clients connected to the Spreadtrum DMA controller must use the format
+described in the dma.txt file, using a two-cell specifier for each channel.
+The two cells in order are:
+1. A phandle pointing to the DMA controller.
+2. The channel id.
+
+spi0: spi@70a00000{
+	...
+	dma-names = "rx_chn", "tx_chn";
+	dmas = <&apdma 11>, <&apdma 12>;
+	...
+};