qcom_mdio: add initial IPQ4018 MDIO support

This adds support for the IPQ4018/IPQ4019 MDIO bus. This is used to
talk to external PHYs and switches. (There's an internal switch
in the IPQ4018/IPQ4019 as well, but it's accessible via MMIO/AXI.)

Differential Revision: https://reviews.freebsd.org/D34110
Reviewed by: manu