iio: adc: ad7192: Add fast settling support

Add fast settling mode support for AD7193.

Add two new device specific attributes: oversampling_ratio and
oversampling_ratio_available.

For AD7193 the user can set the average factor by writing to
oversampling_ratio. The possible values are exposed when reading
oversampling_ratio_available.

Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Link: https://lore.kernel.org/r/20231010124927.143343-4-alisadariana@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>