net: ethernet: mediatek: Extend SGMII related functions

Add SGMII related logic into a separate file, and also provides options for
forcing 1G, 2.5, AN mode for the target PHY, that can be determined from
SGMII node in DTS.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>