dt-bindings: iio: isl29018: convert bindings to YAML format

Convert the isl29018 device tree bindings to the new YAML format.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>