boards/stm32gx: use correct clock base for CLOCK_APBx defines
boards/stm32gx: use correct clock base for CLOCK_APBx defines

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