power_supply: Define Binding for power-supplies

This property is meant to be used in device nodes which represent
power_supply devices that wish to provide a list of supplies which
provide them power, such as a battery listing its chargers.
Signed-off-by: NRhyland Klein <rklein@nvidia.com>
Signed-off-by: NAnton Vorontsov <anton@enomsg.org>

Documentation/devicetree/bindings/power_supply/power_supply.txt

+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>;
+	};