[MM] Power state handling

[Aleksander Morgado <aleksander lanedo com>]

Hey,

I pushed a new 'set-power-state' branch to the upstream MM git repo,
with the changes required to have the power state handled independently,
i.e. low-power not associated to the disabled state.

The branch includes a new 'PowerState' property (exposing the current
power state) and a new 'SetPowerState' method (allowing to change the
power state) in the 'Modem' interface.

With these changes:

  * MM will *never* automatically set in low power mode the modem.

  * MM *may* automatically set in full power mode the modem during the
enabling sequence if it's found to be not in full power mode.

  * Users of the interface can put the modem in low power mode using
SetPowerState(LOW). Users of the interface can also put the modem back
in full power mode using SetPowerState(ON). Note that this method can
only be used while the modem is in 'disabled' state.

A use case of these new items could be an environment implementing
'airplane mode' for the modem devices. When 'airplane mode' is
requested, the modem would need to get 1) disabled and 2) set in low
power state.

Comments and/or review of the branch welcome :)

-- 
Aleksander