I/O channel multiplexer bindings

If a multiplexer is used to select which hardware signal is fed to
e.g. an ADC channel, these bindings describe that situation.

Required properties:
- compatible : "io-channel-mux"
- io-channels : Channel node of the parent channel that has multiplexed
		input.
- io-channel-names : Should be "parent".
- #address-cells = <1>;
- #size-cells = <0>;
- mux-controls : Mux controller node to use for operating the mux
- channels : List of strings, labeling the mux controller states.

For each non-empty string in the channels property, an io-channel will
be created. The number of this io-channel is the same as the index into
the list of strings in the channels property, and also matches the mux
controller state. The mux controller state is described in
../mux/mux-controller.txt

Example:
	mux: mux-controller {
		compatible = "mux-gpio";
		#mux-control-cells = <0>;

		mux-gpios = <&pioA 0 GPIO_ACTIVE_HIGH>,
			    <&pioA 1 GPIO_ACTIVE_HIGH>;
	};

	adc-mux {
		compatible = "io-channel-mux";
		io-channels = <&adc 0>;
		io-channel-names = "parent";

		mux-controls = <&mux>;

		channels = "sync", "in", "system-regulator";
	};