stmmac: dwmac-mediatek: Reuse more common features

This patch makes dwmac-mediatek reuse more features
supported by stmmac_platform.c.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Acked-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>