pm: add new PM_EVENT codes for runtime power transitions

This patch (as1112) adds some new PM_EVENT_* codes for use by kernel
subsystems.  They describe runtime power-state transitions of the sort already
implemented by the USB subsystem.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>