targets:uart: Add uart_3 support

Add uart_3 support, which using GPIO_AD_B1_06 as TX pin, GPIO_AD_B1_07
as RX pin

Signed-off-by: timwang <tim.wang@nxp.com>

Author: Tim-Wang38

targets/TARGET_NXP/TARGET_MCUXpresso_MCUS/TARGET_MIMXRT1050/TARGET_EVK/PeripheralPins.c

@@ -52,11 +52,13 @@ const PinMap PinMap_I2C_SCL[] = {
 /************UART***************/
 const PinMap PinMap_UART_TX[] = {
     {GPIO_AD_B0_12, UART_1, 2},
+    {GPIO_AD_B1_06, UART_3, 2},
     {NC  ,  NC    , 0}
 };
 
 const PinMap PinMap_UART_RX[] = {
     {GPIO_AD_B0_13, UART_1, 2},
+    {GPIO_AD_B1_07, UART_3, 2},
     {NC  ,  NC    , 0}
 };