net: ethernet: mediatek: Re-add support SGMII

* Re-add SGMII support but now with PHYLINK API support
  So the SGMII changes are more clear
* Move SGMII block setup from mtk_gmac_sgmii_path_setup() to
  mtk_mac_config()
* Merge mtk_setup_hw_path() into mtk_mac_config()
* Remove mediatek,physpeed property, fixed-link supports now any speed so
  speed = <2500>; is now valid with PHYLINK
* Demagic SGMII register values
* Use phylink state to setup fixed-link mode

Signed-off-by: René van Dorst <opensource@vdorst.com>
Signed-off-by: David S. Miller <davem@davemloft.net>