Fix order of #includes

This fix modifies the order of system includes to meet the ARM TF coding
standard. There are some exceptions in order to retain header groupings,
minimise changes to imported headers, and where there are headers within
the #if and #ifndef statements.

Change-Id: I65085a142ba6a83792b26efb47df1329153f1624
Signed-off-by: Isla Mitchell <isla.mitchell@arm.com>

lib/psci/psci_system_off.c

@@ -4,12 +4,12 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#include <stddef.h>
 #include <arch_helpers.h>
 #include <assert.h>
 #include <console.h>
 #include <debug.h>
 #include <platform.h>
+#include <stddef.h>
 #include "psci_private.h"
 
 void __dead2 psci_system_off(void)