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>

Documentation/devicetree/bindings/iio/chemical/sensirion,sgp30.txt

+* Sensirion SGP30/SGPC3 multi-pixel Gas Sensor
+
+Required properties:
+
+  - compatible: must be one of
+    "sensirion,sgp30"
+    "sensirion,sgpc3"
+  - reg: the I2C address of the sensor
+
+Example:
+
+gas@58 {
+	compatible = "sensirion,sgp30";
+	reg = <0x58>;
+};