of: Warn if of_graph_get_next_endpoint is called with the root node

If of_graph_get_next_endpoint is given a parentless node instead of an
endpoint node, it is clearly a bug.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>