boards/pic32-wifire: Add CLOCK_CORECLOCK
boards/pic32-wifire: Add CLOCK_CORECLOCK

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

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