[mdio] Add MDIO support for "extended" registers as defined by IEEE 802.3 Clause 45.

IEEE 802.3 Clause 45 added backwards-compatible support for 2^16 PHY registers
through the addition of an additional device address frame.

Clause 45 addressing is used in 10Gbe PHYs, 802.3az EEE registers, etc. It may
make sense to provide a similar extension to the miibus interface, but I've
refrained from unilaterally doing so here.

Submitted by: Landon Fuller <landon@landonf.org>
Differential Revision: https://reviews.freebsd.org/D4607