dt-bindings: net: mediatek-dwmac: add support for mt8188

Add binding document for the ethernet on mt8188

Signed-off-by: Jianguo Zhang <jianguo.zhang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>