dt-bindings: mediatek: Add axi clock in mt8173 dts example

For mt8173, it is needed to add the axi clock for dma mode.

Signed-off-by: Xiangsheng Hou <xiangsheng.hou@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220725120204.10834-1-xiangsheng.hou@mediatek.com

Signed-off-by: Mark Brown <broonie@kernel.org>