iio: adc: ad7768-1: Add filter mode and power mode attributes

Exposing filter mode as an attribute enables the support for
sinc3 and wideband filters and results in more sampling
frequency options. To achieve that, sampling frequency available
attribute now shows a range of values, instead of a list.

The power mode has been decoupled from the filter mode, giving users
independent control via a new IIO attribute. This provides more
flexibility in configuring the ADC's power consumption and performance.

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