stmmac: dwmac-mediatek: add platform level clocks management

This patch implements clks_config callback for dwmac-mediatek platform,
which could support platform level clocks management.

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>