net: stmmac: Add support for external trigger timestamping

The Synopsis MAC controller supports auxiliary snapshot feature that
allows user to store a snapshot of the system time based on an external
event.

This patch add supports to the above mentioned feature. Users will be
able to triggered capturing the time snapshot from user-space using
application such as testptp or any other applications that uses the
PTP_EXTTS_REQUEST ioctl request.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Tan Tee Min <tee.min.tan@intel.com>
Co-developed-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>