power: reset: Add Spreadtrum SC27xx PMIC power off support

On Spreadtrum platform, we need power off system through external SC27xx
series PMICs including the SC2720, SC2721, SC2723, SC2730 and SC2731 chips.
Thus this patch adds SC27xx series PMICs power-off support.

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