iio: adc: ad7606: add offset and phase calibration support

iio: adc: ad7606: add offset and phase calibration support

Add support for offset and phase calibration, only for
devices that support software mode, that are:

ad7606b
ad7606c-16
ad7606c-18

Tested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250606-wip-bl-ad7606-calibration-v9-3-6e014a1f92a2@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>