uart: add fd (file descriptor) to uart context

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

include/mraa_internal_types.h

@@ -92,6 +92,7 @@ struct _uart {
     /*@{*/
     int index; /**< the uart index, as known to the os. */
     char* path; /**< the uart device path. */
+    int fd; /**< file descriptor for device. */
     /*@}*/
 };