power: supply: adc-battery-helper: Add support for optional charge_finished GPIO

power: supply: adc-battery-helper: Add support for optional charge_finished GPIO

Charger ICs often have a status pin which indicates when the charger has
finished charging the battery. Sometimes the status of this pin can be
read over a GPIO.

Add support for optionally reading a charge-finished GPIO and when
available use this to determine when to return POWER_SUPPLY_STATUS_FULL.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://lore.kernel.org/r/20250831122942.47875-5-hansg@kernel.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>