[mdio] migrate mdiobus out of etherswitch and into a top-level device of its own.
The mdio driver interface is generally useful for devices that require MDIO without the full MII bus interface. This lifts the driver/interface out of etherswitch(4), and adds a mdio(4) man page.
Submitted by: Landon Fuller <landon@landonf.org> Differential Revision: https://reviews.freebsd.org/D4606
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