ASoC: jz4740: Add binding documentation for jz4740-i2s

This patch adds binding for the jz4740-i2s driver.
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/ingenic,jz4740-i2s.txt

+Ingenic JZ4740 I2S controller
+
+Required properties:
+- compatible : "ingenic,jz4740-i2s"
+- reg : I2S registers location and length
+- clocks : AIC and I2S PLL clock specifiers.
+- clock-names: "aic" and "i2s"
+- dmas: DMA controller phandle and DMA request line for I2S Tx and Rx channels
+- dma-names: Must be "tx" and "rx"
+
+Example:
+
+i2s: i2s@10020000 {
+	compatible = "ingenic,jz4740-i2s";
+	reg = <0x10020000 0x94>;
+
+	clocks = <&cgu JZ4740_CLK_AIC>, <&cgu JZ4740_CLK_I2SPLL>;
+	clock-names = "aic", "i2s";
+
+	dmas = <&dma 2>, <&dma 3>;
+	dma-names = "tx", "rx";
+
+};