staging: iio: isl29018: document device tree bindings

Fix the following warnings from checkpatch:

WARNING: DT compatible string "isil,isl29018" appears un-documented --
check ./Documentation/devicetree/bindings/
WARNING: DT compatible string "isil,isl29023" appears un-documented --
check ./Documentation/devicetree/bindings/
WARNING: DT compatible string "isil,isl29035" appears un-documented --
check ./Documentation/devicetree/bindings/

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>