iio: adc: Add Spreadtrum SC27XX PMICs ADC support

The Spreadtrum SC27XX PMICs ADC controller contains 32 channels,
which is used to sample voltages with 12 bits conversion.

[Baolin Wang did lots of improvements]

Signed-off-by: Freeman Liu <freeman.liu@spreadtrum.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>