Introduce LS1028A PCI MDIO driver.

Implement support for the NXP LS1028A SoC MDIO controller.
It is attached to the internal PCI root complex.
The controller is used to communicate with PHYs of ports connected
to the internal switch.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30731