Implement a single source file for the following:
* mq2: H2/LPG/CH4/CO/alcohol/smoke/propane/etc
* mq3: Alcohol/benzine/CH4/hexane/LPG/CO
* mq4: CNG (mostly methane)
* mq5: H2/LPG/CH4/CO/alcohol
* mq6: LPG (mostly propane/butane)
* mq7: CO
* mq8: H
* mq9: CO/CH4/LPG
The documentation for these sensors notes that they do not read an exact
gas concentration. Some of the sensors have examples for how to
calibrate with a 'clean air environment' for outputing ppm, but this is
not the case for all mqx sensors. Because of this, the output can be
read as a normalized output (0.0->1.0), raw volts (normalized * aRef),
or a scaled/offset voltage.
Signed-off-by: Noel Eck <noel.eck@intel.com>
