iio: adc: ad4695: implement calibration support

The AD4695 has a calibration feature that allows the user to compensate
for variations in the analog front end. This implements this feature in
the driver using the standard `calibgain` and `calibbias` attributes.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240820-ad4695-gain-offset-v1-2-c8f6e3b47551@baylibre.com


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