fdt: Export the fdt_find_or_add_subnode() function

This function is useful for ensuring that a node exists. Export it so it
can be used more widely.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>