drm/imx: parallel-display: fix drm_panel support

The parallel-display driver used an undocumented, non-standard property
"fsl,panel" to optionally associate with a drm_panel device. This patch
fixes the driver to use the same OF graph bindings as the LDB driver
instead:

    parallel-display {
            compatible = "fsl,imx-parallel-display";
            ...

            port@1 {
                    reg = <1>;

                    parallel_out: endpoint {
                            remote_endpoint = <&panel_in>;
                    };
            };
    };

    panel {
            ...

            port {
                    panel_in: endpoint {
                            remote-endpoint = <&parallel_out>;
                    };
            };
    };

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>