dt-bindings: display: bridge: renesas,dw-hdmi: Add resets property

The DWC HDMI blocks on R-Car and RZ/G2 SoC's use resets, so to complete
the bindings include resets property.

This also fixes the below warning when running dtbs_check:
arch/arm64/boot/dts/renesas/r8a774b1-hihope-rzg2n-rev2-ex-idk-1110wr.dtb: hdmi@fead0000: Unevaluated properties are not allowed ('resets' was unexpected)
	From schema: Documentation/devicetree/bindings/display/bridge/renesas,dw-hdmi.yaml

While at it mark resets property as required as all the DT sources
in the kernel specify resets and update the example node.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>