x86: fsp: Avoid cast stack_top in struct shared_data

Declare stack_top as u32 in struct shared_data and struct common_buf
so that we can avoid casting in fsp_init().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>