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>