iio: add IIO_MASSCONCENTRATION channel type

Measuring particulate matter in ug / m3 (micro-grams per cubic meter)
is de facto standard. Existing air quality sensors usually follow
this convention and are capable of returning measurements using
this unit.

IIO currently does not offer suitable channel type for this
type of measurements hence this patch adds this.

In addition, extra modifiers are introduced used for distinguishing
between fine pm1, pm2p5 and coarse pm4, pm10 particle measurements, i.e
IIO_MOD_PM1, IIO_MOD_PM25 and IIO_MOD_PM4, IIO_MOD_PM10.

pmX consists of particles with aerodynamic diameter less or equal to
X micrometers.

Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>