ASoC: add DT documentation for the sound-name-prefix property

Name prefix:

Card implementing the routing property define the connection between

audio components as list of string pair. Component using the same

sink/source names may use the name prefix property to prepend the

name of their sinks/sources with the provided string.

Optional name prefix property:

- sound-name-prefix : string using as prefix for the sink/source names of

		      the component.

Example: Two instances of the same component.

amp0: analog-amplifier@0 {

	compatible = "simple-audio-amplifier";

	enable-gpios = <&gpio GPIOH_3 0>;

	sound-name-prefix = "FRONT";

};

amp1: analog-amplifier@1 {

	compatible = "simple-audio-amplifier";

	enable-gpios = <&gpio GPIOH_4 0>;

	sound-name-prefix = "BACK";

};