MAINTAINERS: Update gma500 git repo

The GMA500 driver has been handled through drm-misc for a while but the
git repo hasn't been updated. Make sure it points to the right place.

Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921110038.2613604-1-mripard@kernel.org