ARM: dts: imx51: Add IPU ports and endpoints, move imx-drm node to dtsi

This patch connects IPU and and parallel display device tree
nodes using the OF graph bindings described in
Documentation/devicetree/bindings/media/video-interfaces.txt

The IPU ports correspond to the two display interfaces. The
order of endpoints in the ports is arbitrary.

Since the imx-drm node now only needs to contain links to the
display interfaces, it can be moved to the SoC dtsi level. At
the board level, only connections between the display interface
ports and panels have to be added.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>