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>