net: ethernet: mtk-star-emac: add timing adjustment support

Add simple clock inversion for timing adjustment in driver.
Add property "mediatek,txc-inverse" or "mediatek,rxc-inverse" to
device node when necessary.

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>