dt-bindings: iio: adc: add ad7944 ADCs

This adds a new binding for the Analog Devices, Inc. AD7944, AD7985, and
AD7986 ADCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240304-ad7944-mainline-v5-1-f0a38cea8901@baylibre.com


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