power: validate wakeup source before activating it.

A rogue wakeup source not registered in wakeup_sources list is not visible
from wakeup_sources_stats_show. Check if the wakeup source is registered
properly by looking at the timer struct.

Signed-off-by: jinqian <jinqian@google.com>
Change-Id: Id4b2fdb3cf7fecb2fe7576b2305cd337974665c9