iio: adc: ad7192: Correct reference voltage

iio: adc: ad7192: Correct reference voltage

The avdd and the reference voltage are two different sources but the
reference voltage was assigned according to the avdd supply.

Add vref regulator structure and set the reference voltage according to
the vref supply from the devicetree.

In case vref supply is missing, reference voltage is set according to
the avdd supply for compatibility with old devicetrees.

Fixes: b581f748cce0 ("staging: iio: adc: ad7192: move out of staging")
Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230924152149.41884-1-alisadariana@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>