w1-gpio devicetree bindings

Required properties:

 - compatible: "w1-gpio"
 - gpios: one or two GPIO specs:
		- the first one is used as data I/O pin
		- the second one is optional. If specified, it is used as
		  enable pin for an external pin pullup.

Optional properties:

 - linux,open-drain: if specified, the data pin is considered in
		     open-drain mode.

Examples:

	onewire@0 {
		compatible = "w1-gpio";
		gpios = <&gpio 126 0>, <&gpio 105 0>;
	};