dt-bindings: power: supply: axp20x: add AXP813 battery DT binding

The AXP813 can have a battery as power supply, so let's add it to the
list of compatibles.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>