dt-bindings: w1: document generic onewire bindings

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

		     open-drain mode.

Also refer to the generic w1.txt document.

Examples:

	onewire@0 {

	onewire {

		compatible = "w1-gpio";

		gpios = <&gpio 126 0>, <&gpio 105 0>;

	};

		gpios = <&gpio 0 GPIO_ACTIVE_HIGH>;

		battery {

			// ...

		};

	};

Generic devicetree bindings for onewire (w1) busses

===================================================

Onewire busses are described through nodes of their master bus controller.

Slave devices are listed as sub-nodes of such master devices. For now, only

one slave is allowed per bus master.

Example:

	charger: charger {

		compatible = "gpio-charger";

		charger-type = "mains";

		gpios = <&gpio 1 GPIO_ACTIVE_LOW>;

	};

	onewire {

		compatible = "w1-gpio";

		gpios = <&gpio 100 0>, <&gpio 101 0>;

		battery {

			compatible = "maxim,ds2760";

			power-supplies = <&charger>;

		};

	};