ASoC: uniphier: add DT bindings documentation for UniPhier AIO

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>;

	};