staging: imx-drm: Document updated imx-drm device tree bindings

This patch updates the device tree binding documentation for i.MX IPU/display
nodes using the OF graph bindings documented in
Documentation/devicetree/bindings/media/video-interfaces.txt.

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