cpu/stm32gx: factorize HSE clock activation
cpu/stm32gx: factorize HSE clock activation

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