dt-bindings: display: synopsys,dw-hdmi: drop ref from reg-io-width

reg-io-width is a standard property, so no need for defining its type
with $ref.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20220823101031.387082-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>