dt-bindings: usb: mtk-xhci: add support for mt7986

Add compatible string for mt7986.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20221127114142.156573-3-linux@fw-web.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>