Add support for UART hardware flow control

This allows assigning platform pins to the CTS/RTS lines.
If provided, these will be muxed as UART when flow control is enabled.

Signed-off-by: Nicola Lunghi <nicola.lunghi@emutex.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

include/mraa_internal_types.h

@@ -372,6 +372,8 @@ typedef struct {
     unsigned int index; /**< ID as exposed in the system */
     int rx; /**< uart rx */
     int tx; /**< uart tx */
+    int cts; /**< uart cts */
+    int rts; /**< uart rts */
     char* device_path; /**< To store "/dev/ttyS1" for example */
     /*@}*/
 } mraa_uart_dev_t;