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/plat/mediatek/mt8173/drivers/spm/spm.c b/plat/mediatek/mt8173/drivers/spm/spm.c
index 7c6d72b..2801925 100644
--- a/plat/mediatek/mt8173/drivers/spm/spm.c
+++ b/plat/mediatek/mt8173/drivers/spm/spm.c
@@ -55,9 +55,9 @@
 
 DEFINE_BAKERY_LOCK(spm_lock);
 
-static int spm_hotplug_ready __attribute__ ((section("tzfw_coherent_mem")));
-static int spm_mcdi_ready __attribute__ ((section("tzfw_coherent_mem")));
-static int spm_suspend_ready __attribute__ ((section("tzfw_coherent_mem")));
+static int spm_hotplug_ready __section("tzfw_coherent_mem");
+static int spm_mcdi_ready __section("tzfw_coherent_mem");
+static int spm_suspend_ready __section("tzfw_coherent_mem");
 
 void spm_lock_init(void)
 {