ARM: dts: vf610-colibri: enable display controller

Enable dcu node which is used by the DCU DRM driver. Assign the 5.7"
EDT panel with VGA resolution which Toradex sells often with the
evaluation board.

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