RTC timer is default disabled after power off and bootup again. it will be
enabled in kernel rtc driver init. But rtc time is shorter than system clock,
so rtc time cannot update to system clock in rtc_hctosys(), and the sysfs
file /sys/class/rtc/rtc0/hctosys cat result is 0. Android AlarmManagerService
cannot work normally when hctosys is 0.
Enable RTC in u-boot so the time in RTC timer is longer than system clock.
Signed-off-by: Zhang Bo <email@example.com>