Hello,
You can set mac to mac connection or through a phy for auto-negotiation.
u-Boot sets the SerDes to auto-negotiate over SGMII. If the
device at the other end of an SGMII link is configurable to send a
PHY-side response of auto-negotiation type SGMII, then nothing has
to be changed. If the other end has no autoneg support or supports
only the MAC side SGMII auto-negotiation, then see LS1046ARM, Sections 31.8.1.1
and 31.8.1.2.
If you want to use a phy you can refer to both reference design boards (for example AQR113C).