phosphor-hwmon funcitonal interface

[Lei YU]

> Thoughts?  (IIRC< I've mentioned this idea before, but now i'm setting
> up some cycles to hammer it out, as I continue to implement unit-tests
> in phosphor-hwmon)

I second this proposal.

Be noted that this "functional" property on sensor is not the same one on
inventory, and from my understanding, this functional property is not being
used by other services (yet), it's just used to check if hwmon should read the
value for the sensor or not.

So we could re-use this property to indicate if a sensor is functional or not,
especially useful for fans that will get reading error when it's turned off,
e.g. the fans using Aspeed fan/pwm driver.

@Matt What's your idea?