dt-bindings: power: Add Spreadtrum SC27XX fuel gauge unit documentation

This patch adds the binding documentation for Spreadtrum SC27XX series PMICs
fuel gauge unit device, which is used to calculate the battery capacity.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>