MAINTAINERS: Move mxsfb drm driver to drm-misc tree

Another "small driver" moving into drm-misc. Stefan has also offered to
co-maintain it.

Cc: Marek Vasut <marex@denx.de>
Cc: Stefan Agner <stefan@agner.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180919204026.3217-2-sean@poorly.run