POWER_LOSS_PIN for BTT SKR mini MZ / E3 2.0 (#20069)

Marlin/src/pins/stm32f1/pins_BTT_SKR_MINI_E3_common.h

@@ -62,6 +62,13 @@
   #define FIL_RUNOUT_PIN                    PC15  // "E0-STOP"
 #endif
 
+//
+// Power-loss Detection
+//
+#ifndef POWER_LOSS_PIN
+  #define POWER_LOSS_PIN                    PC12  // Power Loss Detection: PWR-DET
+#endif
+
 //
 // Steppers
 //