MAINTAINERS: Update entries for Renesas DRM drivers

The rcar-du has never been maintained in drm-misc. So exclude only
this driver from drm-misc. Also, add the tree entry for sh_mobile.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be> # shmob_drm
Link: https://lore.kernel.org/r/20240218164840.57662-5-biju.das.jz@bp.renesas.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>