igb: add support for 1512 PHY

This patch adds support for Marvell PHY 1512 (required for I354).

Submitted by: Maciej Szwed <maciej.szwed@intel.com>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>