MAINTAINERS: Move fsl-dcu driver to drm-misc tree

The driver is mostly in maintenance mode. Using drm-misc is a
good fit and should make maintenance a bit easier.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926125034.4095-1-stefan@agner.ch