dt-bindings: iio: adc: ad7606: Add iio backend bindings

Add the required properties for iio-backend support, as well as an
example and the conditions to mutually exclude interruption and
conversion trigger with iio-backend.
The iio-backend's function is to controls the communication, and thus the
interruption pin won't be available anymore.
As a consequence, the conversion pin must be controlled externally since
we will miss information about when every single conversion cycle (i.e
conversion + data transfer) ends, hence a PWM is introduced to trigger
the conversions.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-2-654faf1ae08c@baylibre.com


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