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>