fe310: Make header guard unique for plic vendor header
fe310: Make header guard unique for plic vendor header

Otherwise it conflicts with the other plic.h header

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