iio: core: add ADC delay calibration definition

ADCs as ad7606 implement a phase calibration as a delay. Add such
definition, needed for ad7606.

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-v9-2-6e014a1f92a2@baylibre.com


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