staging: iio: adc: ad7192: Add low_pass_3db_filter_frequency

By adding this option we are able to remove the sync3 field and dt binding.
When setting the required cutoff frequency we also determine the ADC
configuration for chop and sync filter.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>