global_data.h: Change ram_top type to phys_addr_t

It's possible to have ram_top above 4 GiB in a 32-bit system, hence
we need to declare ram_top as `phys_addr_t`.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>