power: supply: Add Spreadtrum SC2731 charger support

This patch adds the SC2731 PMIC switch charger support.

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