global: Move from bi_memstart/memsize -> gd->ram_base/ram_size

With the planned removal of bi_memstart & bi_memsize, this patch now
moves the references to the better suiting gd->ram_base/ram_size
variables.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>