igc: Use Start of Packet signal from PHY for timestamping

For better accuracy, i225 is able to do timestamping using the Start of
Packet signal from the PHY.

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>