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>