power: Refactors the code which prints suspend time.

Time for which device suspended is no longer tied to the persistent
clocks. This is specially an issue for architecture which do not
implement read_persistent_clocks()

Bug: 22928771
Change-Id: Ibbaec51819ddad8d38a169237390077b4307022d
Signed-off-by: Ruchi <Kandoi&lt;kandoiruchi@google.com>