dt-bindings: power: Add Spreadtrum SC2731 charger documentation

This patch adds the binding documentation for Spreadtrum SC2731 charger
device.

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>