libfdt: Export overlay_apply_node() as fdt_overlay_apply_node()

This function is useful to merge a subset of DT into another DT, for
example if some prior-stage firmware passes a DT fragment to U-Boot
and U-Boot needs to merge it into its own DT. Export this function
to permit implementing such functionality.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>