iio: chemical: sgp30: Support Sensirion SGP30/SGPC3 sensors

Support Sensirion SGP30 and SGPC3 multi-pixel I2C gas sensors

Supported Features:

* Indoor Air Quality (IAQ) concentrations for
  - tVOC (in_concentration_voc_input)
  - CO2eq (in_concentration_co2_input) - SGP30 only

  IAQ concentrations are periodically read out by a background thread
  to allow the sensor to maintain its internal baseline.

* Gas concentration signals
  - Ethanol (in_concentration_ethanol_raw)
  - H2 (in_concentration_h2_raw) - SGP30 only

https://www.sensirion.com/file/datasheet_sgp30
https://www.sensirion.com/file/datasheet_sgpc3



Signed-off-by: Andreas Brauchli <andreas.brauchli@sensirion.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>