dm: core: add ofnode_get_path()

Add function for retrieving full node path of a given ofnode.
This uses np->full_name if OF is live, otherwise a call to
fdt_get_path() is made.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>