dt-bindings: iio: ad779x: Add binding document

New binding documentation for AD799x series of I²C ADC ICs.

Signed-off-by: Florian Boor <florian.boor@kernelconcepts.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210930104249.2924336-2-florian.boor@kernelconcepts.de


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