Add FAN_MAX_PWM for M106-controlled fans

Marlin/src/inc/Conditionals_post.h

@@ -979,6 +979,23 @@
  */
 #define HAS_FANMUX PIN_EXISTS(FANMUX0)
 
+/**
+ * MIN/MAX fan PWM scaling
+ */
+#ifndef FAN_MIN_PWM
+  #define FAN_MIN_PWM 0
+#endif
+#ifndef FAN_MAX_PWM
+  #define FAN_MAX_PWM 255
+#endif
+#if FAN_MIN_PWM < 0 || FAN_MIN_PWM > 255
+  #error "FAN_MIN_PWM must be a value from 0 to 255."
+#elif FAN_MAX_PWM < 0 || FAN_MAX_PWM > 255
+  #error "FAN_MAX_PWM must be a value from 0 to 255."
+#elif FAN_MIN_PWM > FAN_MAX_PWM
+  #error "FAN_MIN_PWM must be less than or equal to FAN_MAX_PWM."
+#endif
+
 /**
  * Bed Probe dependencies
  */