igb: offer a PTP Hardware Clock instead of the timecompare method

This commit removes the legacy timecompare code from the igb driver and
offers a tunable PHC instead.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>