dt-bindings:iio:frequency:adi,adf4350: txt to yaml format conversion.

This is a large but fairly simple binding.
It may well be possible to constrain some of the properties more than
currently done, but that would involve diving into datasheets for the
supported parts.  Hence for this initial conversion just use the
information that was in the txt file.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Link: https://lore.kernel.org/r/20201031184854.745828-46-jic23@kernel.org