iio: adc: ad7192: Add aincom supply

AINCOM should actually be a supply. AINx inputs are referenced to AINCOM
in pseudo-differential operation mode. AINCOM voltage represents the
offset of corresponding channels.

Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Link: https://lore.kernel.org/r/20240514120222.56488-4-alisa.roman@analog.com


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