cpu/stm32: fix periph_rtt
cpu/stm32: fix periph_rtt

For some reason rtt_get_alarm was never implemented. This adds the
missing function.

OlegHahm/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT