igc: Export a way to read the PTP timer

The next patch will need a way to retrieve the current timestamp from
the NIC's PTP clock.

The 'i225' suffix is removed, if anything model specific is needed,
those specifics should be hidden by this function.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>