Added option to increase the FAN PWM frequency.

Marlin/Marlin.h

@@ -170,6 +170,10 @@ bool IsStopped();
 void enquecommand(const char *cmd); //put an ascii command at the end of the current buffer.
 void prepare_arc_move(char isclockwise);
 
+#ifdef FAST_PWM_FAN
+void setPwmFrequency(uint8_t pin, int val);
+#endif
+
 #ifndef CRITICAL_SECTION_START
   #define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
   #define CRITICAL_SECTION_END    SREG = _sreg;