drm: renesas: rz-du: Add Kconfig dependency between RZG2L_DU and RZG2L_MIPI_DSI

Add Kconfig dependency between RZG2L_DU and RZG2L_MIPI_DSI, so that
DSI module has functional dependency on DU. It is similar way that
the R-Car MIPI DSI encoder is handled.

While at it drop ARCH_RENESAS dependency as DRM_RZG2L_DU depend on
ARCH_RZG2L.

Suggested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240827163727.108405-1-biju.das.jz@bp.renesas.com