iio: adc: ad4695: add offload-based oversampling support

iio: adc: ad4695: add offload-based oversampling support

Add support for the ad4695's oversampling feature when SPI offload is
available. This allows the ad4695 to set oversampling ratios on a
per-channel basis, raising the effective-number-of-bits from 16
(OSR == 1) to 17 (4), 18 (16), or 19 (64) for a given sample (i.e. one
full cycle through the auto-sequencer). The logic for reading and
writing sampling frequency for a given channel is also adjusted based on
the current oversampling ratio.

The non-offload case isn't supported as there isn't a good way to
trigger the CNV pin in this mode. Support could be added in the future
if a use-case arises.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250109-ad4695-oversampling-v2-1-a46ac487082c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>