Remove direct usage of __attribute__((foo))

Migrate all direct usage of __attribute__ to usage of their
corresponding macros from cdefs.h.
e.g.:
 - __attribute__((unused)) -> __unused

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
diff --git a/include/bl31/runtime_svc.h b/include/bl31/runtime_svc.h
index 87f2dd2..03f906e 100644
--- a/include/bl31/runtime_svc.h
+++ b/include/bl31/runtime_svc.h
@@ -88,7 +88,7 @@
  */
 #define DECLARE_RT_SVC(_name, _start, _end, _type, _setup, _smch) \
 	static const rt_svc_desc_t __svc_desc_ ## _name \
-		__attribute__ ((section("rt_svc_descs"), used)) = { \
+		__section("rt_svc_descs") __used = { \
 			.start_oen = _start, \
 			.end_oen = _end, \
 			.call_type = _type, \