[NUCLEO_F401RE] Put back SERIAL_TX and SERIAL_RX pins

Even if it is a duplication of USB_TX/RX pins, it is better to have them
available.

Author: bcostm

libraries/mbed/targets/hal/TARGET_STM/TARGET_NUCLEO_F401RE/PinNames.h

@@ -150,6 +150,8 @@ typedef enum {
     LED3        = PA_5,
     LED4        = PA_5,
     USER_BUTTON = PC_13,
+    SERIAL_TX   = PA_2,
+    SERIAL_RX   = PA_3,
     USBTX       = PA_2,
     USBRX       = PA_3,
     I2C_SCL     = PB_8,