dt-bindings: power: supply: axp20x-battery: Add monitored-battery

Document the monitored-battery property, which the existing driver can
use to set certain properties.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Link: https://lore.kernel.org/r/20240821215456.962564-8-macroalpha82@gmail.comSigned-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

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

@@ -28,6 +28,12 @@ properties:
           - const: x-powers,axp813-battery-power-supply
       - const: x-powers,axp813-battery-power-supply
 
+  monitored-battery:
+    description:
+      Specifies the phandle of an optional simple-battery connected to
+      this gauge.
+    $ref: /schemas/types.yaml#/definitions/phandle
+
 required:
   - compatible