boards/6lowpan-clicker: Add CLOCK_CORECLOCK
boards/6lowpan-clicker: Add CLOCK_CORECLOCK

Also sneak in use of MHZ() macro, so that reviews don't need to count
zeros.

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