Power Supply Core Support

Optional Properties:
 - power-supplies : This property is added to a supply in order to list the
   devices which supply it power, referenced by their phandles.

Example:

	usb-charger: power@e {
		compatible = "some,usb-charger";
		...
	};

	ac-charger: power@c {
		compatible = "some,ac-charger";
		...
	};

	battery@b {
		compatible = "some,battery";
		...
		power-supplies = <&usb-charger>, <&ac-charger>;
	};