drivers: sensor: Add support for CCS811 Digital Gas Sensor

Add support for AMS CCS811 Digital Gas Sensor for monitoring
indoor air quality. This sensor reports the following parameters:

1. Co2 concentration
2. VOC concentration
3. Sensor voltage
4. Sensor current

This driver only supports polling mode as of now.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>