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/drivers/io/io_fip.c b/drivers/io/io_fip.c
index d291423..99cf15b 100644
--- a/drivers/io/io_fip.c
+++ b/drivers/io/io_fip.c
@@ -128,7 +128,7 @@
 
 
 /* Open a connection to the FIP device */
-static int fip_dev_open(const uintptr_t dev_spec __attribute__((unused)),
+static int fip_dev_open(const uintptr_t dev_spec __unused,
 			 io_dev_info_t **dev_info)
 {
 	assert(dev_info != NULL);