dt-bindings: power: supply: gpio-charger: convert to yaml

Convert the gpio-charger bindings from text format to
new YAML based representation.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>