HAL_SITL: added uartD

libraries/AP_HAL_AVR_SITL/HAL_AVR_SITL_Class.cpp

@@ -39,14 +39,16 @@ static Empty::EmptySPIDeviceManager emptySPI;
 static SITLUARTDriver sitlUart0Driver(0, &sitlState);
 static SITLUARTDriver sitlUart1Driver(1, &sitlState);
 static SITLUARTDriver sitlUart2Driver(2, &sitlState);
+static SITLUARTDriver sitlUart3Driver(3, &sitlState);
 
 static SITLUtil utilInstance;
 
 HAL_AVR_SITL::HAL_AVR_SITL() :
     AP_HAL::HAL(
 	    &sitlUart0Driver,  /* uartA */
-        &sitlUart1Driver, /* uartB */
+        &sitlUart1Driver,  /* uartB */
         &sitlUart2Driver,  /* uartC */
+        &sitlUart3Driver,  /* uartD */
         &emptyI2C, /* i2c */
         &emptySPI, /* spi */
         &sitlAnalogIn, /* analogin */