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_memmap.c b/drivers/io/io_memmap.c
index d45107e..ff4efa8 100644
--- a/drivers/io/io_memmap.c
+++ b/drivers/io/io_memmap.c
@@ -95,7 +95,7 @@
 
 
 /* Open a connection to the memmap device */
-static int memmap_dev_open(const uintptr_t dev_spec __attribute__((unused)),
+static int memmap_dev_open(const uintptr_t dev_spec __unused,
 			   io_dev_info_t **dev_info)
 {
 	assert(dev_info != NULL);