dt-bindings: power: supply: axp20x-battery: Add AXP209 compatible

The AXP209 compatible was used in Device Trees and the driver, but it
was never documented.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: linux-pm@vger.kernel.org
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/supply/x-powers,axp20x-battery-power-supply.yaml

@@ -21,6 +21,7 @@ properties:
   compatible:
     oneOf:
       - const: x-powers,axp202-battery-power-supply
+      - const: x-powers,axp209-battery-power-supply
       - const: x-powers,axp221-battery-power-supply
       - items:
           - const: x-powers,axp803-battery-power-supply