phy: mdio-octeon: Refactor into two files/modules

A follow-on patch uses PCI probing to find the Thunder MDIO hardware.
In preparation for this, split out the common code into a new file
mdio-cavium.c, which will be used by both the existing OCTEON driver,
and the new Thunder PCI based driver.

As part of the refactoring simplify the struct cavium_mdiobus by
removing fields that are only ever used in the probe function and can
just as well be local variables.

Use readq/writeq in preference to readq_relaxed/writeq_relaxed as the
relaxed form was an optimization for an early chip revision, and the
MDIO drivers are not performance bottlenecks that need optimization in
the first place.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>