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/arm/lib/Makefile b/arch/arm/lib/Makefile
index d74e4b8..da8ed72 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -35,6 +35,7 @@
 obj-$(CONFIG_SEMIHOSTING) += semihosting.o
 
 obj-y	+= sections.o
+obj-y	+= stack.o
 ifdef CONFIG_ARM64
 obj-y	+= gic_64.o
 obj-y	+= interrupts_64.o