ASoC: dt-bindings: realtek,rt5616: Document audio graph port

ASoC: dt-bindings: realtek,rt5616: Document audio graph port

The codec can be used in conjunction with an audio-graph-card to provide
an endpoint for binding with the other side of the audio link.

Document the 'port' property that is used for this to prevent
dtbscheck errors like:

    rockchip/rk3588-nanopc-t6-lts.dtb: codec@1b: Unevaluated properties are not allowed ('port' was unexpected)
        from schema $id: http://devicetree.org/schemas/sound/realtek,rt5616.yaml#

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20240830203819.1972536-3-heiko@sntech.de
Signed-off-by: Mark Brown <broonie@kernel.org>