iio: adc: sc27xx: refactor some functions for support more PMiCs

Refactor the common adc_nvmem_cell_calib_data,adc_to_volt and call
these in the origin sc27xx_adc_scale_calibration,sc27xx_adc_to_volt

Signed-off-by: Cixi Geng <cixi.geng1@unisoc.com>
Link: https://lore.kernel.org/r/20220419142458.884933-6-gengcixi@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>