Fix the renesas,du binding 'ports' schema which is referencing the 'port'
schema instead of the 'ports' schema.

Fixes: 99d66127 ("dt-bindings: display: renesas,du: Convert binding to YAML")
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20210623164308.2570164-1-robh@kernel.org