dt-bindings: ASoC: Add Spreadtrum DMA platform documentation

Add documentation for Spreadtrum DMA platform driver.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/sprd-pcm.txt

+* Spreadtrum DMA platfrom bindings
+
+Required properties:
+- compatible: Should be "sprd,pcm-platform".
+- dmas: Specify the list of DMA controller phandle and DMA request line ordered pairs.
+- dma-names: Identifier string for each DMA request line in the dmas property.
+  These strings correspond 1:1 with the ordered pairs in dmas.
+
+Example:
+
+	audio_platform:platform@0 {
+		compatible = "sprd,pcm-platform";
+		dmas = <&agcp_dma 1 1>, <&agcp_dma 2 2>,
+		     <&agcp_dma 3 3>, <&agcp_dma 4 4>,
+		     <&agcp_dma 5 5>, <&agcp_dma 6 6>,
+		     <&agcp_dma 7 7>, <&agcp_dma 8 8>,
+		     <&agcp_dma 9 9>, <&agcp_dma 10 10>;
+		dma-names = "normal_p_l", "normal_p_r",
+			"normal_c_l", "normal_c_r",
+			"voice_c", "fast_p",
+			"loop_c", "loop_p",
+			"voip_c", "voip_p";
+	};