dt-bindings: power: supply: bq256xx: Add missing power-supply ref

Reference the common power supply schema to bring the definition of
monitored-battery property.  Drop also the obvious description because
it is duplicating what is in power-supply.yaml.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250604-dt-bindings-psy-monitored-battery-v1-2-7f755ff75218@linaro.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/supply/bq256xx.yaml

@@ -58,9 +58,7 @@ properties:
     minimum: 100000
     maximum: 3200000
 
-  monitored-battery:
-    $ref: /schemas/types.yaml#/definitions/phandle
-    description: phandle to the battery node being monitored
+  monitored-battery: true
 
   interrupts:
     maxItems: 1
@@ -78,6 +76,7 @@ required:
   - monitored-battery
 
 allOf:
+  - $ref: power-supply.yaml#
   - if:
       properties:
         compatible: