ASoC: img: Add binding document for SPDIF input controller

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

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

+Imagination Technologies SPDIF Input Controller
+
+Required Properties:
+
+  - compatible : Compatible list, must contain "img,spdif-in"
+
+  - #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:
+	"rx"
+
+  - clocks : Contains an entry for each entry in clock-names
+
+  - clock-names : Includes the following entries:
+	"sys"	The system clock
+
+Optional Properties:
+
+  - resets: Should contain a phandle to the spdif in reset signal, if any
+
+  - reset-names: Should contain the reset signal name "rst", if a
+	reset phandle is given
+
+  - interrupts : Contains the spdif in interrupt, if present
+
+Example:
+
+spdif_in: spdif-in@18100E00 {
+	compatible = "img,spdif-in";
+	reg = <0x18100E00 0x100>;
+	interrupts = <GIC_SHARED 20 IRQ_TYPE_LEVEL_HIGH>;
+	dmas = <&mdc 15 0xffffffff 0>;
+	dma-names = "rx";
+	clocks = <&cr_periph SYS_CLK_SPDIF_IN>;
+	clock-names = "sys";
+	#sound-dai-cells = <0>;
+};