x86: link: Set up device tree for SPL

Add the correct pre-relocation tag so that the required device tree nodes
are present in the SPL device tree.

On x86 it doesn't make a lot of sense to have a separate SPL device tree.
Since everything is in the same ROM we might as well just use the main
device tree in both SPL and U-Boot proper. But we haven't implemented that,
so this is a good first step.

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