STM32 CAN3: add CAN_3 declaration for DISCO_F769NI

Author: bcostm

targets/TARGET_STM/TARGET_STM32F7/TARGET_STM32F769xI/TARGET_DISCO_F769NI/PeripheralNames.h

@@ -94,7 +94,8 @@ typedef enum {
 
 typedef enum {
     CAN_1 = (int)CAN1_BASE,
-    CAN_2 = (int)CAN2_BASE
+    CAN_2 = (int)CAN2_BASE,
+    CAN_3 = (int)CAN3_BASE
 } CANName;
 
 #ifdef __cplusplus