dt-bindings: iio: Add Spreadtrum SC27XX PMICs ADC controller documentation

This patch adds the binding documentation for Spreadtrum SC27XX series
PMICs ADC controller device.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>