iio: adc: ad7124: Implement system calibration

Allow triggering both zero-scale and full-scale calibration via sysfs in
the same way as it's done for ad7173.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/20250303114659.1672695-18-u.kleine-koenig@baylibre.com


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