drm: exynos: dsi: Lookup OF-graph or Child node devices

In general, for MIPI DSI there are three ways to represent the
pipeline for an upstream bridge to find the connected downstream
panel or bridge.

1. Child panel or bridge as a conventional device tree child node.
2. Child panel or bridge as an OF-graph port node.
3. Child panel or bridge as an OF-graph ports node.

There are three different downstream panels or bridges that are
possible to connect an upstream DSI host bridge - DSI Panel,
DSI Bridge, and I2C-Configured DSI bridge.

An example of the downstream panel represented as a child node,

&dsi {
   compatible = "samsung,exynos5433-mipi-dsi";

   ports {
        port@0 {
             reg = <0>;

             dsi_to_mic: endpoint {
                  remote-endpoint = <&mic_to_dsi>;
             };
        };
   };

   panel@0 {
        reg = <0>;
   };
};

An example of the downstream bridge represented as a port node,

&i2c4 {
   bridge@2c {
	compatible = "ti,sn65dsi84";

        ports {
             port@0 {
                  reg = <0>;

                  bridge_in_dsi: endpoint {
                       remote-endpoint = <&dsi_out_bridge>;
                       data-lanes = <1 2>;
                  };
             };

	     port@2 {
	          reg = <2>;

                  bridge_out_panel: endpoint {
                       remote-endpoint = <&panel_out_bridge>;
                  };
             };
        };
   };
};

&dsi {
   compatible = "fsl,imx8mm-mipi-dsim";

   port {
        dsi_in_lcdif: endpoint@0 {
	     reg = <0>;
	     remote-endpoint = <&lcdif_out_dsi>;
	};

	dsi_out_bridge: endpoint@1 {
	     reg = <1>;
	     remote-endpoint = <&bridge_in_dsi>;
	};
   };
};

An example of the downstream bridge represented as a ports node,

&dsi {
   compatible = "fsl,imx8mm-mipi-dsim";

   ports {
	port@0 {
	     reg = <0>;

	     dsi_in_lcdif: endpoint@0 {
		  reg = <0>;
		  remote-endpoint = <&lcdif_out_dsi>;
	     };
	};

	port@1 {
	     reg = <1>;

	     dsi_out_bridge: endpoint {
		  remote-endpoint = <&bridge_in_dsi>;
	     };
	};
};

In, summary it is possible to represent all three downstream slaves
devices using OF-graph port or ports node however only DSI Panel and
DSI Bridge are possible but not possible to represent I2C-Configured
DSI bridge child nodes since I2C-Configure bridges are child of I2C
node, not upstream DSI host bridge and it is must represent them
endpoint port linking.

This indeed means, the OF-graph port or ports representation is
mandatory for I2C-Configured DSI bridges.

This patch tries to add an OF-graph port or ports representation
detection code on top of existing child node detection.

It is possible to replace the entire detection code using existing
drm_of helper drm_of_find_panel_or_bridge but it will break the
Exynos DSI since the pipeline doesn't support OF-graph port or ports
node.

Overall, this patch has a combination of child and OF-graph pipeline
detections in order to support the backward compatibility of Exynos
DSI child node and i.MX8M Mini/Nano/Plus OF-graph port or ports
node pipelines.

This is the first common DSI host bridge driver that needs to support
all possible downstream connection pipeline combinations.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>