iio: chemical: Add driver support for sgp40

sgp40 is a gas sensor used for measuring the air quality.

This driver is reading the raw resistance value which can be passed to
an userspace algorithm for further calculation.

The raw value is also used to calculate an estimated absolute voc index
in the range from 0 to 500. For this purpose the raw_mean value of the
resistance for which the index value is 250 might be set up as a
calibration step. This can be done with in_resistance_calibbias.

Compensation of relative humidity and temperature is supported and can
be used by writing to output values of out_humidityrelative_raw and
out_temp_raw.

There is a predecesor sensor type (sgp30) already existing. This driver
module was not extended because the new sensor is quite different in its
i2c telegrams.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20210804154641.GA3237@arbad
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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