dt-bindings: mfd: Add Spreadtrum SC27xx PMIC documentation

This patch adds the binding documentation for Spreadtrum SC27xx series
PMIC device.

Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>