cpu/fe310: Add used registers to clobbers
cpu/fe310: Add used registers to clobbers

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