MAINTAINERS: Update DRM DRIVERS FOR FREESCALE IMX entry

As per the discussion on the lists [1], changes to this driver
generally flow through drm-misc. If they need to be coordinated with
v4l2 they sometimes go through Philipp Zabel's tree instead. List both
trees in MAINTAINERS. Also update the title of this driver to specify
that it's just for IMX 5/6 since, as per Philipp "There are a lot more
i.MX that do not use IPUv3 than those that do."

[1] https://lore.kernel.org/r/d56dfb568711b4b932edc9601010feda020c2c22.camel@pengutronix.de

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906072803.1.Idef7e77e8961cbeb8625183eec9db0356b2eccd0@changeid