ASoC: add audio-graph-scu-card document

Audio-Graph-SCU-Card:

Audio-Graph-SCU-Card is "Audio-Graph-Card" + "ALSA DPCM".

It is based on common bindings for device graphs.

see ${LINUX}/Documentation/devicetree/bindings/graph.txt

Basically, Audio-Graph-SCU-Card property is same as

Simple-Card / Simple-SCU-Card / Audio-Graph-Card.

see ${LINUX}/Documentation/devicetree/bindings/sound/simple-card.txt

    ${LINUX}/Documentation/devicetree/bindings/sound/simple-scu-card.txt

    ${LINUX}/Documentation/devicetree/bindings/sound/audio-graph-card.txt

Below are same as Simple-Card / Audio-Graph-Card.

- label

- dai-format

- frame-master

- bitclock-master

- bitclock-inversion

- frame-inversion

- dai-tdm-slot-num

- dai-tdm-slot-width

- clocks / system-clock-frequency

Below are same as Simple-SCU-Card.

- convert-rate

- convert-channels

- prefix

- routing

Required properties:

- compatible				: "audio-graph-scu-card";

- dais					: list of CPU DAI port{s}

Example

	sound_card {

		compatible = "audio-graph-scu-card";

		label = "sound-card";

		prefix = "codec";

		routing = "codec Playback", "DAI0 Playback",

			"codec Playback", "DAI1 Playback";

		convert-rate = <48000>;

		dais = <&cpu_port>;

	};

	audio-codec {

		...

		port {

			codec_endpoint: endpoint {

				remote-endpoint = <&cpu_endpoint>;

			};

		};

	};

	dai-controller {

		...

		cpu_port: port {

			cpu_endpoint: endpoint {

				remote-endpoint = <&codec_endpoint>;

				dai-format = "left_j";

				...

			};

		};

	};