dt-bindings: document gpio-charger bindings

This documents the binding for the gpio-charger power-supply.
Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>

Documentation/devicetree/bindings/power_supply/gpio-charger.txt

+gpio-charger
+
+Required properties :
+ - compatible : "gpio-charger"
+ - gpios : GPIO indicating the charger presence.
+   See GPIO binding in bindings/gpio/gpio.txt .
+ - charger-type : power supply type, one of
+     unknown
+     battery
+     ups
+     mains
+     usb-sdp (USB standard downstream port)
+     usb-dcp (USB dedicated charging port)
+     usb-cdp (USB charging downstream port)
+     usb-aca (USB accessory charger adapter)
+
+Example:
+
+	usb_charger: charger {
+		compatible = "gpio-charger";
+		charger-type = "usb-sdp";
+		gpios = <&gpf0 2 0 0 0>;
+	}
+
+	battery {
+		power-supplies = <&usb_charger>;
+	};