net: enetc: Initialize SerDes for SGMII and USXGMII protocols

ENETC has ethernet MACs capable of SGMII, 2500BaseX and USXGMII. But in
order to use these protocols some SerDes configurations need to be
performed. The SerDes is configurable via an internal PCS PHY which is
connected to an internal MDIO bus at address 0.

This patch basically removes the dependency on bootloader regarding
SerDes initialization.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>