imx-drm: parallel-display: Add drm_panel support

imx-drm: parallel-display: Add drm_panel support

This patch allows to optionally attach the parallel-display to a panel
supported by a drm_panel driver instead of supplying the modes via
device tree.

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

		display-timings {
			native-timing = <&timing1>;
			timing1: etm0700g0dh6 {
				hactive = <800>;
				vactive = <480>;
				clock-frequency = <33260000>;
				hsync-len = <128>;
				hback-porch = <88>;
				hfront-porch = <40>;
				vsync-len = <2>;
				vback-porch = <33>;
				vfront-porch = <10>;
				hsync-active = <0>;
				vsync-active = <0>;
				...
			};
		};
		...
	};

After:
	parallel-display {
		compatible = "fsl,imx-parallel-display";
		fsl,panel = <&panel>;
		...
	};

	panel: panel {
		compatible = "edt,etm0700g0dh6", "simple-panel";
	};

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>