drm: renesas: Move RZ/G2L MIPI DSI driver to rz-du

All the RZ/G2L DU specific components are located under the rz-du folder,
so it makes sense to move the RZ/G2L MIPI DSI driver there instead of
keeping it in the rcar-du folder. This change improves the organization
and modularity of the driver configuration by grouping related settings together.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240625123244.200533-1-prabhakar.mahadev-lad.rj@bp.renesas.com