cpu: RTC implementation for STM32F1
cpu: RTC implementation for STM32F1

Works get_time, set_time, alarm and wakeup after set power mode STM32_PM_STOP

aeneby/RIOT,josephnoir/RIOT,aeneby/RIOT,josephnoir/RIOT,ant9000/RIOT,toonst/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,x3ro/RIOT,aeneby/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT,jasonatran/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,x3ro/RIOT,toonst/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,josephnoir/RIOT,kYc0o/RIOT,jasonatran/RIOT,josephnoir/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,smlng/RIOT,toonst/RIOT,RIOT-OS/RIOT,ant9000/RIOT,yogo1212/RIOT,basilfx/RIOT,aeneby/RIOT,authmillenon/RIOT,basilfx/RIOT,OTAkeys/RIOT,smlng/RIOT,authmillenon/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,mtausig/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,yogo1212/RIOT,OTAkeys/RIOT,yogo1212/RIOT,ant9000/RIOT,toonst/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,x3ro/RIOT,mtausig/RIOT,aeneby/RIOT,smlng/RIOT,basilfx/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,basilfx/RIOT,x3ro/RIOT,smlng/RIOT,basilfx/RIOT,yogo1212/RIOT,mtausig/RIOT,smlng/RIOT,miri64/RIOT,ant9000/RIOT,x3ro/RIOT,authmillenon/RIOT,kYc0o/RIOT,mtausig/RIOT,mtausig/RIOT