Allows user to set (almost) any PWM frequency (#12638)

Marlin/src/inc/Conditionals_post.h

@@ -1213,6 +1213,13 @@
   #error "FAN_MIN_PWM must be less than or equal to FAN_MAX_PWM."
 #endif
 
+/**
+ * FAST PWM FAN Settings
+ */
+#if ENABLED(FAST_PWM_FAN) && !defined(FAST_PWM_FAN_FREQUENCY)
+  #define FAST_PWM_FAN_FREQUENCY ((F_CPU) / (2 * 255 * 1)) // Fan frequency default
+#endif
+
 /**
  * Bed Probe dependencies
  */