iio: adc: ad7124: add filter support

iio: adc: ad7124: add filter support

Add support to the ad7124 driver for selecting the filter type.

The filter type has an influence on the effective sampling frequency of
each channel. For sinc3+pf{1,2,3,4}, the sampling frequency is fixed.
For sinc{3,4} (without post filter), there is a factor of 3 or 4
depending on the filter type. For the extra +sinc1, there is an extra
averaging factor that depends on the power mode.

In order to select the closest sampling frequency for each filter type,
we keep a copy of the requested sampling frequency. This way, if the
user sets the sampling frequency first and then selects the filter type,
the sampling frequency will still be as close as possible to the
requested value.

Since we always either have the SINGLE_CYCLE bit set or have more than
one channel enabled, the sampling frequency is always using the
"zero-latency" calculation from the data sheet. This is only documented
for the basic sinc{3,4} filters, so the other filter types had to be
inferred and confirmed through testing.

Since the flat filter type list consists of multiple register fields,
the struct ad7124_channel_config::filter_type field is changed to the
enum ad7124_filter_type type to avoid nested switch statements in a
lot of places.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>