dt-bindings: iio: frequency: Add ADMV4420 doc

Add device tree bindings for the ADMV4420 K band downconverter.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220223130808.13352-1-cristian.pop@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>