Introduce MDIO read/write functions for LS1028A.

Provide common MDIO code for two LS1028 ENETC controllers -
an external one found on the PCI bus and internal one found in ENETC.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30730