igc: Add skeletal frame for Intel(R) 2.5G Ethernet Controller support

This patch adds the beginning framework onto which I am going to add
the igc driver which supports the Intel(R) I225-LM/I225-V 2.5G
Ethernet Controller.

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