ASoC: img: Add binding document for SPDIF output controller

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

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

+Imagination Technologies SPDIF Output Controller
+
+Required Properties:
+
+  - compatible : Compatible list, must contain "img,spdif-out"
+
+  - #sound-dai-cells : Must be equal to 0
+
+  - reg : Offset and length of the register set for the device
+
+  - dmas: Contains an entry for each entry in dma-names.
+
+  - dma-names: Must include the following entry:
+	"tx"
+
+  - clocks : Contains an entry for each entry in clock-names.
+
+  - clock-names : Includes the following entries:
+	"sys"	The system clock
+	"ref"	The reference clock
+
+  - resets: Contains a phandle to the spdif out reset signal
+
+  - reset-names: Contains the reset signal name "rst"
+
+Optional Properties:
+
+  - interrupts : Contains the parallel out interrupt, if present
+
+Example:
+
+spdif_out: spdif-out@18100D00 {
+	compatible = "img,spdif-out";
+	reg = <0x18100D00 0x100>;
+	interrupts = <GIC_SHARED 21 IRQ_TYPE_LEVEL_HIGH>;
+	dmas = <&mdc 14 0xffffffff 0>;
+	dma-names = "tx";
+	clocks = <&cr_periph SYS_CLK_SPDIF_OUT>,
+		 <&clk_core CLK_SPDIF>;
+	clock-names = "sys", "ref";
+	resets = <&pistachio_reset PISTACHIO_RESET_SPDIF_OUT>;
+	reset-names = "rst";
+	#sound-dai-cells = <0>;
+};