dt-bindings: staging: iio: cdc: ad7746: add binding documentation for AD7746

Add device tree binding documentation for AD7746 cdc in YAML format.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/2c2f4d486a94e0740e112bfac0d9306bdb7ea69c.1619841953.git.lucas.p.stankus@gmail.com


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