igc: Add initial EEE support

IEEE802.3az-2010 Energy Efficient Ethernet has been
approved as standard (September 2010) and the driver
can enable and disable it via ethtool.
Disable the feature by default on parts which support it.
Add enable/disable eee options.
tx-lpi, tx-timer and advertise not supported yet.

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