igb: add support for SGMII-based MDIO PHYs

This patch adds support for external MDIO PHYs, in addition to
the standard SFP support for SGMII PHYs over the I2C interface.

Signed-off-by: Nicholas Nunley <nicholas.d.nunley@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>