Power: add an API to log wakeup reasons

Add API log_wakeup_reason() and expose it to userspace via sysfs path
/sys/kernel/wakeup_reasons/last_resume_reason

Change-Id: I5be7f1183b368c968cbde6f8504e74ff68e76ab4
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>