AP_SerialManager: added a new MSP DisplayPort serial backend

libraries/AP_SerialManager/AP_SerialManager.cpp

@@ -524,6 +524,7 @@ void AP_SerialManager::init()
 #if HAL_MSP_ENABLED                    
                 case SerialProtocol_MSP:
                 case SerialProtocol_DJI_FPV:
+                case SerialProtocol_MSP_DisplayPort:
                     // baudrate defaults to 115200
                     state[i].baud.set_default(AP_SERIALMANAGER_MSP_BAUD/1000);
                     uart->begin(map_baudrate(state[i].baud),

libraries/AP_SerialManager/AP_SerialManager.h

@@ -155,6 +155,7 @@ public:
         SerialProtocol_Torqeedo = 39,
         SerialProtocol_AIS = 40,
         SerialProtocol_CoDevESC = 41,
+        SerialProtocol_MSP_DisplayPort = 42,
         SerialProtocol_NumProtocols                    // must be the last value
     };