[power] Add PM usage counting

[power] Add PM usage counting

- Add ->pm_users and ->pm_parent fields to struct dev_pm_info.
- Add function device_pm_set_parent()

The default power parent for a device is the device's physical parent, but
a driver may change it to represent a tranversal power dependency.

Though it's not incorporated into the suspend/resume sequences yet, the
core will respect the power tree, rather than the physical/electrical one.

Also added is a power usage count for devices, which indicates how many
devices are dependent on that one for power (how many children it has in
the power tree). The core will use this count to determine whether or not
a device can be put into a low power state or not.