igb: allow setting MAC address on i211 using a device tree blob

The Intel i211 LOM PCIe Ethernet controllers' iNVM operates as an OTP
and has no external EEPROM interface [1]. The following allows the
driver to pickup the MAC address from a device tree blob when CONFIG_OF
has been enabled.

[1]
http://www.intel.com/content/www/us/en/embedded/products/networking/i211-ethernet-controller-datasheet.html



Signed-off-by: John Holland <jotihojr@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>