net: ethernet: mtk-star-emac: add support for MII interface

Add support for MII interface.
If user wants to use MII, assign "MII" to "phy-mode" property in dts.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Yinghua Pan <ot_yinghua.pan@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>