iio: chemical: sgp30: add device tree support

Add device tree support for Sensirion SGP30/SGPC3 gas sensors.

Signed-off-by: Andreas Brauchli <andreas.brauchli@sensirion.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>