dt-bindings: iio: chemical: Add bindings for bme680

BME680 is a pressure/temperature/humidity/voc sensor.
Signed-off-by: Sebastien Bourdelin <sebastien.bourdelin@gmail.com>
Acked-by: Himanshu Jha <himanshujha199640@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/chemical/bme680.txt

+Bosch Sensortec BME680 pressure/temperature/humidity/voc sensors
+
+Required properties:
+- compatible: must be "bosch,bme680"
+
+Example:
+
+bme680@76 {
+          compatible = "bosch,bme680";
+          reg = <0x76>;
+};