ixgbe: Add 1000Base-T device based on X550EM_X MAC

Add support for new 1000Base-T device based on X550EM_X MAC
type. All PHY operations are disabled as the PHY is controlled
by FW.

Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>