power: supply: sc27xx: Add one property to read charge voltage

Add POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE property to get charge
voltage sampling by ADC controller, which is used to validate if the
charge voltage is in normal range or not in charger manager.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>