net: ethernet: mtk-star-emac: add clock pad selection for RMII

This patch add a new dts property named "mediatek,rmii-rxc" parsing
in driver, which will configure MAC to select which pin the RMII reference
clock is connected to, TXC or RXC.

TXC pad is the default reference clock pin. If user wants to use RXC pad
instead, add "mediatek,rmii-rxc" to corresponding device node.

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>