mii_physubr: Add support for limiting PHY max speed

In some cases we might want to limit the max speed advertised below of what
the PHY is capable of.
This is usually the case when we connect 1G capable PHY to 100M MAC, or when
some exotic physical connection is used.
Add a new mii_maxspeed field to mii_softc and parse it in mii_phy_dev_attach.
Speed limit is normally located in DT.
The property is already parsed in mii_fdt.c, but its value still has to be
passed by the PHY driver.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D32727