power: supply: Add Spreadtrum SC27XX fuel gauge unit driver

This patch adds the Spreadtrum SC27XX serial PMICs fuel gauge support,
which is used to calculate the battery capacity.

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