Add powerOff support for MeshPocket

variants/mesh_pocket/MeshPocket.h

@@ -41,5 +41,9 @@ public:
     NVIC_SystemReset();
   }
 
+  void powerOff() override {
+    sd_power_system_off();
+  }
+
   bool startOTAUpdate(const char* id, char reply[]) override;
 };