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.

• Add all existing mdio(4) clients (and dependencies) to the kernel config (note that this excludes if_mge (ARM-specific) and e6000sw (not built in current tree):

device etherswitch
device arswitch
device ip17x
device rtl8366rb
device ukswitch
device miiproxy
device iicbus

• Verify that the kernel still builds.