iio: adc: sc27xx: structure adjustment and optimization

iio: adc: sc27xx: structure adjustment and optimization

Introduce one variant device data structure to be compatible
with SC2731 PMIC since it has different scale and ratio calculation
and so on. also rename the SC27xx to SC2731 for some definition
and function.

Co-developed-by: Yuming Zhu <yuming.zhu1@unisoc.com>
Signed-off-by: Yuming Zhu <yuming.zhu1@unisoc.com>
Signed-off-by: Cixi Geng <cixi.geng1@unisoc.com>
Link: https://lore.kernel.org/r/20220419142458.884933-5-gengcixi@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>