iio: frequency: admv4420.c: Add support for ADMV4420

Add support for K Band Downconverter with Integrated
Fractional-N PLL and VCO.
More info:
https://www.analog.com/en/products/admv4420.html

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20220223130808.13352-2-cristian.pop@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>