iio: chemical: bme680: Add device-tree support

This commit allow the driver to work with device-tree.

Signed-off-by: Sebastien Bourdelin <sebastien.bourdelin@gmail.com>
Acked-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>