iio: frequency: adrf6780: add support for ADRF6780

iio: frequency: adrf6780: add support for ADRF6780

The ADRF6780 is a silicon germanium (SiGe) design, wideband,
microwave upconverter optimized for point to point microwave
radio designs operating in the 5.9 GHz to 23.6 GHz frequency
range.

Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/ADRF6780.pdf

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20211021113244.56936-1-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>