ASoC: simple-audio-mux: add state-labels

ASoC: simple-audio-mux: add state-labels

Merge series from Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>:

simple-audio-mux is designed to be used generally, thus "Input 1" or
"Input 2" are used to selecting MUX input. This numbered inputs would work,
but might be not user friendly in some case, for example in case of system
hardware design has some clear labels.
Adds new "state-labels" property and enable to select MUX by own state names.

Original
	> amixer set "MUX" "Input 1"
	> amixer set "MUX" "Input 2"

Use mux-names
	sound_mux: mux {
		compatible = "simple-audio-mux";
		mux-gpios = <...>;
=>		state-labels = "Label_A", "Label_B";
	};

	> amixer set "MUX" "Label_A"
	> amixer set "MUX" "Label_B"