x86: fsp: Set up init runtime buffer in update_fsp_configs()

fsp_init() runtime buffer parameter might be different across
different platforms. Move this to update_fsp_configs().

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