igc: Add NVM support

Add code for NVM support and get MAC address, complete probe
method.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>