iio: adc: ad4062: Add IIO Trigger support

iio: adc: ad4062: Add IIO Trigger support

Adds support for IIO Trigger. Optionally, gp1 is assigned as Data Ready
signal, if not present, fallback to an I3C IBI with the same role.
The software trigger is allocated by the device, but must be attached by
the user before enabling the buffer. The purpose is to not impede
removing the driver due to the increased reference count when
iio_trigger_set_immutable() or iio_trigger_get() is used.

Signed-off-by: Jorge Marques <jorge.marques@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>