Make use of user/system includes more consistent

Make codebase consistent in its use of #include "" syntax for
user includes and #include <> syntax for system includes.

Fixes ARM-software/tf-issues#65

Change-Id: If2f7c4885173b1fd05ac2cde5f1c8a07000c7a33
diff --git a/drivers/io/io_semihosting.c b/drivers/io/io_semihosting.c
index 14ec687..c8013bc 100644
--- a/drivers/io/io_semihosting.c
+++ b/drivers/io/io_semihosting.c
@@ -29,9 +29,9 @@
  */
 
 #include <assert.h>
-#include "io_storage.h"
-#include "io_driver.h"
-#include "semihosting.h"
+#include <io_storage.h>
+#include <io_driver.h>
+#include <semihosting.h>