igc: Add support for ethtool GET_TS_INFO command

This command allows igc to report what types of timestamping are
supported. ptp4l uses this to detect if the hardware supports
timestamping.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>