dt-bindings: iio: frequency: add adrf6780 doc

Add device tree bindings for the ADRF6780 Upconverter.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20211021113244.56936-2-antoniu.miclaus@analog.com


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