dt-bindings: iio: chemical: Add binding for CCS811 VOC sensor

This commit adds devicetree binding for AMS CCS811 VOC sensor.

Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>