ASoC: img: Add binding document for I2S output controller

Add binding document for Imagination Technologies I2S output
controller
Signed-off-by: Damien.Horsley <Damien.Horsley@imgtec.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/img,i2s-out.txt

+Imagination Technologies I2S Output Controller
+
+Required Properties:
+
+  - compatible : Compatible list, must contain "img,i2s-out"
+
+  - #sound-dai-cells : Must be equal to 0
+
+  - reg : Offset and length of the register set for the device
+
+  - clocks : Contains an entry for each entry in clock-names
+
+  - clock-names : Must include the following entries:
+	"sys"	The system clock
+	"ref"	The reference clock
+
+  - dmas: Contains an entry for each entry in dma-names.
+
+  - dma-names: Must include the following entry:
+	"tx"	Single DMA channel used by all active I2S channels
+
+  - img,i2s-channels : Number of I2S channels instantiated in the I2S out block
+
+  - resets: Contains a phandle to the I2S out reset signal
+
+  - reset-names: Contains the reset signal name "rst"
+
+Optional Properties:
+
+  - interrupts : Contains the I2S out interrupts. Depending on
+	the configuration, there may be no interrupts, one interrupt,
+	or an interrupt per I2S channel. For the case where there is
+	one interrupt per channel, the interrupts should be listed
+	in ascending channel order
+
+Example:
+
+i2s_out: i2s-out@18100A00 {
+	compatible = "img,i2s-out";
+	reg = <0x18100A00 0x200>;
+	interrupts = <GIC_SHARED 13 IRQ_TYPE_LEVEL_HIGH>;
+	dmas = <&mdc 23 0xffffffff 0>;
+	dma-names = "tx";
+	clocks = <&cr_periph SYS_CLK_I2S_OUT>,
+		 <&clk_core CLK_I2S>;
+	clock-names = "sys", "ref";
+	img,i2s-channels = <6>;
+	resets = <&pistachio_reset PISTACHIO_RESET_I2S_OUT>;
+	reset-names = "rst";
+	#sound-dai-cells = <0>;
+};