uart: add function mraa_uart_get_dev_path

Used for getting the path to the character device under linux for uart
device. i.e. "/dev/ttyS0"

Adds paths to existing platforms.
Closes #84

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

api/mraa/common.h

@@ -149,6 +149,7 @@ typedef struct {
     unsigned int index; /**< ID as exposed in the system */
     int rx; /**< uart rx */
     int tx; /**< uart tx */
+    char* device_path; /**< To store "/dev/ttyS1" for example */
     /*@}*/
 } mraa_uart_dev_t;