Add support for 56000 baud on nrf51

Add support for the baudrate 56000.  This is not documented in the
datasheet, but Nordic has indicated in can be used by setting the
BAUD register to 56000.

Author: c1728p9

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/serial_api.c

@@ -39,6 +39,7 @@ static const int acceptedSpeeds[17][2] = {
     {28800, UART_BAUDRATE_BAUDRATE_Baud28800},
     {31250, (0x00800000UL) /* 31250 baud */},
     {38400, UART_BAUDRATE_BAUDRATE_Baud38400},
+    {56000, (0x00E51000UL) /* 56000 baud */},
     {57600, UART_BAUDRATE_BAUDRATE_Baud57600},
     {76800, UART_BAUDRATE_BAUDRATE_Baud76800},
     {115200, UART_BAUDRATE_BAUDRATE_Baud115200},