Merge branch 'mv88e6xxx-external-MDIO'

Andrew Lunn says:

====================
External MDIO support for mv88e6xxx

The mv88e6390 family of switches has two MDIO busses, one internal to
the switch and a second one for external usage. Older generations of switches
have a single MDIO bus, which is used both internally and externally.

Refactor the existing MDIO driver code to allow for multiple MDIO
busses, and implement the second MDIO bus on mv88e6390.

This is a rewrite of a patch previously submitted as part of "Batch
3". It has been broken up into 5 smaller patches. A compatible string
is now used in the device tree to indicate the external MDIO bus.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>