drivers: net: apply serdes configuration for ENETC Ethernet interfaces

Ethernet interfaces using serial protocols go through the serdes block
integrated in the SoC.  This is accessed over dedicated internal MDIOs
which are part of the Ethernet PCI functions.  Set up serdes at _start,
along with other protocol specific port/MAC configuration.
MDIO code is shared with enetc_mdio, read/write functions are exported
from fsl_enetc_mdio for this reason.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>