ixgbe: register a mdiobus

Most dsa devices expect a 'struct mii_bus' pointer to talk to switches
via the MII interface.

While this works for dsa devices, it will not work safely with Linux
PHYs in all configurations since the firmware of the ixgbe device may
be polling some PHY addresses in the background.

Signed-off-by: Stephen Douthit <stephend@silicom-usa.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>