PM: sleep: Switch to rtc_time64_to_tm()/rtc_tm_to_time64()

Call the 64bit versions of rtc_tm time conversion to avoid the y2038 issue.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>