eth: mtk-eth: enable mt7629 sgmii mode support in mediatek eth driver

The sgmii mode init flow is almost the same for all mediatek SoC, the
only difference is the register offset(SGMSYS_GEN2_SPEED) is 0x2028
in the old chip(mt7622) but changed to 0x128 for the newer chip(mt7629
and the following chips).

Signed-off-by: MarkLee <Mark-MC.Lee@mediatek.com>