iio: adc: ad4695: implement triggered buffer

This implements buffered reads for the ad4695 driver using the typical
triggered buffer implementation, including adding a soft timestamp
channel.

The chip has 4 different modes for doing conversions. The driver is
using the advanced sequencer mode since that is the only mode that
allows individual configuration of all aspects each channel (e.g.
bipolar config currently and oversampling to be added in the future).
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240813-iio-adc-ad4695-buffered-read-v2-1-9bb19fc1924b@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>