iio: adc: Add driver for ADS7128 / ADS7138

iio: adc: Add driver for ADS7128 / ADS7138

Add driver for ADS7128 and ADS7138 12-bit, 8-channel analog-to-digital
converters. These ADCs have a wide operating range and a wide feature
set. Communication is based on the I2C interface.
ADS7128 differs in the addition of further hardware features, like a
root-mean-square (RMS) and a zero-crossing-detect (ZCD) module.

Signed-off-by: Tobias Sperling <tobias.sperling@softing.com>
Link: https://patch.msgid.link/20250213-adc_ml-v4-2-66b68f8fdb8c@softing.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>