iio: chemical: Add IIO_CONCENTRATION channel type

There are air quality sensors that report data back in parts per million
of VOC (Volatile Organic Compounds) which are usually indexed from CO2
or another common pollutant.

This patchset adds an IIO_CONCENTRATION type that returns a percentage
of substance because no other channels types fit this use case.
Modifiers for IIO_MOD_CO2 and IIO_MOD_VOC gas types are defined.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>