of: property: Document that of_graph_get_endpoint_by_regs needs of_node_put

The node returned by of_graph_get_endpoint_by_regs has a reference taken,
and we need to put that reference back when done with the node.

However, the documentation for that node doesn't mention it, so let's make
sure it does.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>