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 <[e-mail]>
Signed-off-by: Chris Morgan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sebastian Reichel <[e-mail]>

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

···
           - 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