iio: adc: ad4170-4: Add digital filter and sample frequency config support

Add support for sinc3, sinc5, and averaged sinc5 digital filters along with
sample frequency configuration.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/31f4226b4172b0bbb26daa054b74b25b1966c7b2.1751895245.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: machschmitt