Hibernation: Update messages

Make hibernation messages start with one common prefix "PM: " and use
the word "hibernation" in the messages as a synonym of "suspend to
disk".

Turn some KERN_INFO messages into debug ones.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Len Brown <len.brown@intel.com>