dt-bindings: iio: chemical: Add trivial DT binding for sgp40

Add devicetree binding for Sensirion sgp40 gas sensor to trivial
devices.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Link: https://lore.kernel.org/r/20210804154549.GA3223@arbad


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>