post, memorytest: add support for none powerpc archs

change bd->bi_memsize to gd->ram_size, as this is defined
on all archs, so this post test can used on none powerpc
archs too.

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <hs@denx.de>
cc: Mike Frysinger <vapier@gentoo.org>