Device tree bindings for i2c-cbus-gpio driver

Required properties:
	- compatible = "i2c-cbus-gpio";
	- gpios: clk, dat, sel
	- #address-cells = <1>;
	- #size-cells = <0>;

Optional properties:
	- child nodes conforming to i2c bus binding

Example:

i2c@0 {
	compatible = "i2c-cbus-gpio";
	gpios = <&gpio 66 0 /* clk */
		 &gpio 65 0 /* dat */
		 &gpio 64 0 /* sel */
		>;
	#address-cells = <1>;
	#size-cells = <0>;

	retu-mfd: retu@1 {
		compatible = "retu-mfd";
		reg = <0x1>;
	};
};