dt-bindings: iio: adc: adi,ad7606: add gain calibration support

Add gain calibration support by a per-channel resistor value.

Acked-by: Conor Dooley <conor.dooley@microchip.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-v9-4-6e014a1f92a2@baylibre.com


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