fix: promicro: add powerOff

variants/promicro/PromicroBoard.h

@@ -61,5 +61,9 @@ public:
     NVIC_SystemReset();
   }
   
+  void powerOff() override {
+    sd_power_system_off();
+  }
+
   bool startOTAUpdate(const char* id, char reply[]) override;
 };