iio: adc: ad7768-1: add filter mode and decimation rate attributes

Separate filter mode and decimation rate from the sampling frequency
attribute. The new filter mode attribute enables SINC3 and WIDEBAND
filters, which were previously unavailable.

Previously, combining decimation and MCLK divider in the sampling
frequency obscured performance trade-offs. Lower MCLK divider
settings increase power usage, while lower decimation rates reduce
precision by decreasing averaging. By creating a decimation attribute,
users gain finer control over performance.

The addition of those attributes allows a wider range of sampling
frequencies and more access to the device features.

Co-developed-by: PopPaul2021 <paul.pop@analog.com>
Signed-off-by: PopPaul2021 <paul.pop@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>

Author: jonathanns