AArch32: Enable override of plat_set_my_stack/plat_get_my_stack

This patch makes the default MP definitions of plat_get_my_stack()
and plat_set_my_stack() as weak so that they can be overridden by
the AArch32 Secure Payload if it requires.

Change-Id: I3b6ddff5750443a776505e3023ff2934227592b6
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

plat/common/aarch32/platform_mp_stack.S

 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -32,8 +32,8 @@
 #include <asm_macros.S>
 #include <platform_def.h>
 
-	.globl	plat_get_my_stack
-	.globl	plat_set_my_stack
+	.weak	plat_get_my_stack
+	.weak	plat_set_my_stack
 
 	/* -----------------------------------------------------
 	 * uintptr_t plat_get_my_stack (u_register_t mpidr)