cpu/fe310: add Kconfig configuration
cpu/fe310: add Kconfig configuration

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