cpu/cc2538: add RTT_MIN_OFFSET
cpu/cc2538: add RTT_MIN_OFFSET

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