iio: adc: ad_sigma_delta: Add optional irq selection

Add optional irq_num attribute to ad_sigma_delta_info structure for
selecting the used interrupt line for ADC's conversion completion.

Signed-off-by: Dumitru Ceclan <mitrutzceclan@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240228110622.25114-2-mitrutzceclan@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>