ARM: dts: vf610-colibri: use of_graph dt nodes to describe the panel

The fsl,panel property is deprecated and we should use the new
of_graph dt bindings to describe the relationship between the DCU
controller and the panel.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>