Add link to F_CPU_ACTUAL vs F_CPU explanation

teensy4/clockspeed.c

 #include "wiring.h"
 #include "debug/printf.h"
 
+// A brief explanation of F_CPU_ACTUAL vs F_CPU
+//  https://forum.pjrc.com/threads/57236?p=212642&viewfull=1#post212642
 volatile uint32_t F_CPU_ACTUAL = 396000000;
 volatile uint32_t F_BUS_ACTUAL = 132000000;