PM / sleep: Mark suspend/hibernation start and finish

Regardless of whether or not debug messages from the core system
suspend/hibernation code are enabled, it is useful to know when
system-wide transitions start and finish (or fail), so print "info"
messages at these points.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mark Salyzyn <salyzyn@android.com>