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_sgpc3Signed-off-by: Andreas Brauchli <andreas.brauchli@sensirion.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/chemical/Makefile

@@ -9,5 +9,6 @@ obj-$(CONFIG_BME680_I2C) += bme680_i2c.o
 obj-$(CONFIG_BME680_SPI) += bme680_spi.o
 obj-$(CONFIG_CCS811)		+= ccs811.o
 obj-$(CONFIG_IAQCORE)		+= ams-iaq-core.o
+obj-$(CONFIG_SENSIRION_SGP30)	+= sgp30.o
 obj-$(CONFIG_SPS30) += sps30.o
 obj-$(CONFIG_VZ89X)		+= vz89x.o