spl: Makefile: include /config in the (reduced) FDT used by the SPL stage

When OF control is enabled for the SPL stage, nodes are removed from
the DTB to reduce its size. While /chosen is kept, /config is removed.

There's no reason why /chosen should be kept over /config (and as we
would like to put properties into /config that control the SPL stage),
we add '/config' to the list of nodes to be retained for the SPL stage.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>