iio: adc: adi-axi-adc: support modified prbs23

Add support for configuring the prbs23 sequence.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-4-4cb62852f0d0@analog.com


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