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.com


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>