iio: Add initial module

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

include/mraa_internal_types.h

@@ -129,6 +129,13 @@ struct _uart {
     /*@}*/
 };
 
+/**
+ * A structure reprensting an IIO device
+ */
+struct _iio {
+    int num; /**< IIO device number */
+};
+
 /**
  * A bitfield representing the capabilities of a pin.
  */