rtc: fsl-ftm-alarm: switch to ktime_get_real_seconds

The driver drops the nanoseconds part of the timespec64, there is no need
to call ktime_get_real_ts64.

Link: https://lore.kernel.org/r/20191016201223.30568-2-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>