Modern Intel systems supports cross timestamping of the network device
clock and Always Running Timer (ART) in hardware.  This allows the
device time and system time to be precisely correlated. The timestamp
pair is returned through e1000e_phc_get_syncdevicetime() used by
get_system_device_crosststamp().  The hardware cross-timestamp result
is made available to applications through the PTP_SYS_OFFSET_PRECISE
ioctl which calls e1000e_phc_getcrosststamp().

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: kevin.b.stanton@intel.com
Cc: kevin.j.clarke@intel.com
Cc: hpa@zytor.com
Cc: jeffrey.t.kirsher@intel.com
Cc: netdev@vger.kernel.org
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
[jstultz: Reworked to use new interface, commit message tweaks]
Signed-off-by: John Stultz <john.stultz@linaro.org>