iio:as3935: Add DT binding docs for AS3935 driver

Document compatible string, required and optional DT properties for
AS3935 chipset driver.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>