power: supply: sc27xx: Save last battery capacity

Our charger manager can optimize the battery capacity periodically, so
we can save last battery capacity into registers. Then next system
power-on, we can read the last saved battery capacity as the initial
battery capacity, which can make the battery capacity more accurate.

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