ASoC: uniphier: add DT bindings documentation for UniPhier AIO

This patch adds DT binding documentation for UniPhier
AIO audio subsystem.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Katsuhiro Suzuki <suzuki.katsuhiro@socionext.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/uniphier,aio.txt

+Socionext UniPhier SoC audio driver
+
+The Socionext UniPhier audio subsystem consists of I2S and S/PDIF blocks in
+the same register space.
+
+Required properties:
+- compatible      : should be one of the following:
+		    "socionext,uniphier-ld11-aio"
+		    "socionext,uniphier-ld20-aio"
+- reg             : offset and length of the register set for the device.
+- interrupts      : should contain I2S or S/PDIF interrupt.
+- pinctrl-names   : should be "default".
+- pinctrl-0       : defined I2S signal pins for an external codec chip.
+- clock-names     : should include following entries:
+                    "aio"
+- clocks          : a list of phandle, should contain an entry for each
+                    entry in clock-names.
+- reset-names     : should include following entries:
+                    "aio"
+- resets          : a list of phandle, should contain an entry for each
+                    entry in reset-names.
+- #sound-dai-cells: should be 1.
+
+Example:
+	audio {
+		compatible = "socionext,uniphier-ld20-aio";
+		reg = <0x56000000 0x80000>;
+		interrupts = <0 144 4>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_aout>;
+		clock-names = "aio";
+		clocks = <&sys_clk 40>;
+		reset-names = "aio";
+		resets = <&sys_rst 40>;
+		#sound-dai-cells = <1>;
+	};