power: supply: max17040: Add POWER_SUPPLY_PROP_CAPACITY_ALERT_MIN

Adds the property POWER_SUPPLY_PROP_CAPACITY_ALERT_MIN to export the
chip->low_soc_alert and add the property as writeable, implementing
max17040_prop_writeable and max17040_set_property, so with that the
user space can readjust the alerts.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>