Fix incorrect pin in PinMap_UART_RX - issue #1657 (#1675)

PTC6 was wrong, should be PTD6

Author: Colin Hogben

libraries/mbed/targets/hal/TARGET_Freescale/TARGET_KPSDK_MCUS/TARGET_MCU_K64F/TARGET_FRDM/PeripheralPins.c

@@ -108,7 +108,7 @@ const PinMap PinMap_UART_RX[] = {
     {PTC3 , UART_1, 3},
     {PTC14, UART_4, 3},
     {PTD2 , UART_2, 3},
-    {PTC6 , UART_0, 3},
+    {PTD6 , UART_0, 3},
     {NC  ,  NC    , 0}
 };