🔨 Robin Nano V3 overridable POWER_LOSS_PIN (#22123)

Marlin/src/pins/stm32f4/pins_MKS_ROBIN_NANO_V3.h

   #define FIL_RUNOUT2_PIN               MT_DET_2
 #endif
 
-#define POWER_LOSS_PIN                    PW_DET
+#ifndef POWER_LOSS_PIN
+  #define POWER_LOSS_PIN                  PW_DET
+#endif
 #define PS_ON_PIN                         PW_OFF
 
 //