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>
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index 0f62982..05b22bb 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -40,6 +40,7 @@
 obj-y	+= interrupts.o
 obj-$(CONFIG_CMD_KGDB) += kgdb.o
 obj-$(CONFIG_CMD_IDE) += ide.o
+obj-y	+= stack.o
 obj-y	+= time.o
 
 # Don't include the MPC5xxx special memcpy into the