dt-bindings:iio:proximity:ams,as3935 yaml conversion

A straight forward conversion of this binding. I have added
a maximum SPI frequency from the datasheet.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20201031134110.724233-12-jic23@kernel.org