iio: frequency: admfm2000: New driver

Dual microwave down converter module with input RF and LO frequency
ranges from 0.5 to 32 GHz and an output IF frequency range from 0.1 to
8 GHz. It consists of a LNA, mixer, IF filter, DSA, and IF amplifier
for each down conversion path.

Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Link: https://lore.kernel.org/r/20240123081059.5746-2-kimseer.paller@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>