MAINTAINERS: Add IPUv3 core driver to the i.MX DRM driver section

The ipu-v3 core driver is maintained in and merged through
the same trees as the imx-drm drivers that are using it.
This patch adds the drivers/gpu/ipu-v3/ directory to the
i.MX DRM driver section.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>