dt-bindings: power: supply: axp20x: Add AXP803 compatible

The AXP803 compatible was introduced recently with a fallback to the
AXP813, 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>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>