RTC: rtc-mrst: follow on to the change of rtc_device_register()

commit f44f7f96 (RTC: Initialize kernel state from RTC) will
call rtc_read_alarm() inside rtc_device_register(), so rtc-mrst
driver need to call dev_set_drvdata() before rtc_device_register()
get called.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>