dt-bindings: power: supply: Add charge-status-gpios property

Add documentation for the "charge-status-gpios" property.
Update the "gpios" property with a valid example.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/supply/gpio-charger.txt

@@ -14,12 +14,16 @@ Required properties :
      usb-cdp (USB charging downstream port)
      usb-aca (USB accessory charger adapter)
 
+Optional properties:
+ - charge-status-gpios: GPIO indicating whether a battery is charging.
+
 Example:
 
 	usb_charger: charger {
 		compatible = "gpio-charger";
 		charger-type = "usb-sdp";
-		gpios = <&gpf0 2 0 0 0>;
+		gpios = <&gpd 28 GPIO_ACTIVE_LOW>;
+		charge-status-gpios = <&gpc 27 GPIO_ACTIVE_LOW>;
 	};
 
 	battery {