drm: rcar-du: Update description for DRM_RCAR_DW_HDMI Kconfig entry

The rcar_dw_hdmi driver is also used on Renesas RZ/G2 SoCs. Update the
Kconfig entry description to reflect this.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>