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>

Documentation/devicetree/bindings/power/supply/axp20x_battery.txt

@@ -4,12 +4,12 @@ Required Properties:
  - compatible, one of:
 			"x-powers,axp209-battery-power-supply"
 			"x-powers,axp221-battery-power-supply"
+			"x-powers,axp813-battery-power-supply"
 
-This node is a subnode of the axp20x/axp22x PMIC.
+This node is a subnode of its respective PMIC DT node.
 
-The AXP20X and AXP22X can read the battery voltage, charge and discharge
-currents of the battery by reading ADC channels from the AXP20X/AXP22X
-ADC.
+The supported devices can read the battery voltage, charge and discharge
+currents of the battery by reading ADC channels from the ADC.
 
 Example: