drivers: Adapt old platforms to periph rtc

I guess the implementation could need some cleanup and more documentation.

NOTE: the TI cc430 can only set alarms based on minutes, hours and days of week - not on seconds, day of month, day of year, month or year.
The discussion in https://github.com/RIOT-OS/RIOT/issues/2046 converged (from  my point of view) to the point that auto_init should not work on peripheral devices.