cgem: support SGMII PHY connection mode

As the PolarFire SoC needs SGMII to connect the PHY, check the
'phy-mode' property of device tree node for ethernet and act on it
appropriately.

Add the compatible strings for the PolarFire SoC device tree.
'microchip,mpfs-mss-gem" is not officially documented but has been
observed in the available firmware for this platform, so it is included
for now.

Also, fix a typo in if_cgem_hw.h.

Reviewed by: mhorne
MFC after: 1 week
Sponsored by: Conclusive Engineering
Differential Revision: https://reviews.freebsd.org/D34764