rtc: fsl-ftm-alarm: switch to rtc_time64_to_tm/rtc_tm_to_time64

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

Link: https://lore.kernel.org/r/20191016201223.30568-3-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>