dt-bindings: display: sun8i-a83t-dw-hdmi: Reference dw-hdmi YAML schema

Replace the reference to the DWC HDMI text DT binding with a reference
to the YAML equivalent.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>