iio: adc: ad7944: add driver for AD7944/AD7985/AD7986

iio: adc: ad7944: add driver for AD7944/AD7985/AD7986

This adds a driver for the Analog Devices Inc. AD7944, AD7985, and
AD7986 ADCs. These are a family of pin-compatible ADCs that can sample
at rates up to 2.5 MSPS.

The initial driver adds support for sampling at lower rates using the
usual IIO triggered buffer and can handle all 3 possible reference
voltage configurations.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240304-ad7944-mainline-v5-2-f0a38cea8901@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>