ARM: dts: rockchip: fix graph node unit address error from dtc

The updated dtc emits a warning for the edp-panel of two rk3288 boards:
    Warning (graph_endpoint): /dp@ff970000/ports/port@1/endpoint: graph node unit address error, expected "0"

Fix this by adding the necessary @0 to the endpoint node.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>