cpu/samd21/cpu: reset GCLK before configuring them
cpu/samd21/cpu: reset GCLK before configuring them

OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,OTAkeys/RIOT,smlng/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,basilfx/RIOT,kaspar030/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,basilfx/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,smlng/RIOT,ant9000/RIOT,yogo1212/RIOT,ant9000/RIOT,miri64/RIOT,smlng/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,smlng/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,yogo1212/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,jasonatran/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,basilfx/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,smlng/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT