common/board_f: factor out reserve_stacks

Introduce arch_reserve_stacks() to tailor gd->start_addr_sp and gd->irq_sp to
the architecture needs.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/powerpc/lib/stack.c

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2015 Andreas Bießmann <andreas.devel@googlemail.com>
+ *
+ * Copyright (c) 2011 The Chromium OS Authors.
+ * (C) Copyright 2002-2006
+ * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
+ *
+ * (C) Copyright 2002
+ * Sysgo Real-Time Solutions, GmbH <www.elinos.com>
+ * Marius Groeger <mgroeger@sysgo.de>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+#include <common.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+int arch_reserve_stacks(void)
+{
+	ulong *s;
+
+	/* setup stack pointer for exceptions */
+	gd->irq_sp = gd->start_addr_sp;
+
+	/* Clear initial stack frame */
+	s = (ulong *)gd->start_addr_sp;
+	*s = 0; /* Terminate back chain */
+	*++s = 0; /* NULL return address */
+
+	return 0;
+}