PM / Runtime: Use device type and device class callbacks

The power management of some devices is handled through device types
and device classes rather than through bus types.  Since these
devices may also benefit from using the run-time power management
core, extend it so that the device type and device class run-time PM
callbacks can be taken into consideration by it if the bus type
callback is not defined.

Update the run-time PM core documentation to reflect this change.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>