iio: adc: ad7173: add calibration support

The ad7173 family of chips has up to four calibration modes.

Internal zero scale: removes ADC core offset errors.
Internal full scale: removes ADC core gain errors.
System zero scale: reduces offset error to the order of channel noise.
System full scale: reduces gain error to the order of channel noise.

All voltage channels will undergo an internal zero/full scale
calibration at bootup.

System zero/full scale can be done after bootup using the newly created
iio interface 'sys_calibration' and 'sys_calibration_mode'

Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>
Link: https://patch.msgid.link/20241202-ad411x_calibration-v3-1-beb6aeec39e2@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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