iio: adc: ad7944: Add support for "3-wire mode"

This adds support for AD7944 ADCs wired in "3-wire mode". (NOTE: 3-wire
is the datasheet name for this wiring configuration and has nothing to
do with SPI_3WIRE.)

In the 3-wire mode, the SPI controller CS line can be wired to the CNV
line on the ADC and used to trigger conversions rather that using a
separate GPIO line.

The turbo/chain mode compatibility check at the end of the probe
function is technically can't be triggered right now but adding it now
anyway so that we don't forget to add it later when support for
daisy-chaining is added.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240314-mainline-ad7944-3-wire-mode-v2-1-d469da0705d2@baylibre.com


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