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/services/std_svc/psci/psci_common.c b/services/std_svc/psci/psci_common.c
index 7332695..465c5fd 100644
--- a/services/std_svc/psci/psci_common.c
+++ b/services/std_svc/psci/psci_common.c
@@ -74,7 +74,7 @@
  ******************************************************************************/
 non_cpu_pd_node_t psci_non_cpu_pd_nodes[PSCI_NUM_NON_CPU_PWR_DOMAINS]
 #if USE_COHERENT_MEM
-__attribute__ ((section("tzfw_coherent_mem")))
+__section("tzfw_coherent_mem")
 #endif
 ;