iio: adc: ad7768-1: add filter type and oversampling ratio attributes

Separate filter type and decimation rate from the sampling frequency
attribute. The new filter type attribute enables sinc3, sinc3+rej60
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 an oversampling
attribute, which controls the decimation, 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. Sampling frequency
table is updated after every digital filter parameter change.

Changes in the sampling frequency are not allowed anymore while in
buffered mode.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Co-developed-by: Pop Paul <paul.pop@analog.com>
Signed-off-by: Pop Paul <paul.pop@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/cd3b60c44847d5c35cecc4385bbda6533be6825e.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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