iio: proximity: as3935: add missing required spi-max-frequency

spi-max-frequency is required to be set for the as3935 part, and
document such usage.

Cc: devicetree@vger.kernel.org
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Rob Herring <robh@kernel.org>