arm, post, memory: fix bug if sdram base != 0x00000000

commit 8d3fcb5e breaks post
memory support for sdram base != 0x00000000. Fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Cc: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>