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>

include/mraa_internal_types.h

@@ -91,5 +91,6 @@ struct _aio {
 struct _uart {
     /*@{*/
     int index; /**< the uart index, as known to the os. */
+    char* path; /**< the uart device path. */
     /*@}*/
 };