igc: Add basic skeleton for PTP

This allows the creation of the /dev/ptpX device for i225, and reading
and writing the time.

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>