iio: adc: ad9467: convert to backend framework

iio: adc: ad9467: convert to backend framework

Convert the driver to use the new IIO backend framework. The device
functionality is expected to be the same (meaning no added or removed
features).

Also note this patch effectively breaks ABI and that's needed so we can
properly support this device and add needed features making use of the
new IIO framework.

Given the lack of features (and devices supported) in the ad9467 driver
compared with the ADI out of tree version, we don't expect any user of
the upstream driver so no one should notice the ABI breakage. However,
if someone is affected by this, ADI will happily support transitioning
to the backend framework.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240210-iio-backend-v11-6-f5242a5fb42a@analog.com
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>