Add RAK4631 support for rs232 bridge

src/helpers/bridges/RS232Bridge.cpp

@@ -15,6 +15,8 @@ void RS232Bridge::begin() {
 
 #if defined(ESP32)
   ((HardwareSerial *)_serial)->setPins(WITH_RS232_BRIDGE_RX, WITH_RS232_BRIDGE_TX);
+#elif defined(RAK_4631) 
+  ((Uart *)_serial)->setPins(WITH_RS232_BRIDGE_RX, WITH_RS232_BRIDGE_TX);
 #elif defined(NRF52_PLATFORM)
   ((HardwareSerial *)_serial)->setPins(WITH_RS232_BRIDGE_RX, WITH_RS232_BRIDGE_TX);
 #elif defined(RP2040_PLATFORM)