ptp: introduce PTP_CLOCK_EXTOFF event for the measured external offset

ptp: introduce PTP_CLOCK_EXTOFF event for the measured external offset

This change is for the PHC devices that can measure the phase offset
between PHC signal and the external signal, such as the 1PPS signal of
GNSS. Reporting PTP_CLOCK_EXTOFF to user space will be piggy-backed to
the existing ptp_extts_event so that application such as ts2phc can
poll the external offset the same way as extts. Hence, ts2phc can use
the offset to achieve the alignment between PHC and the external signal
by the help of either SW or HW filters.

Signed-off-by: Min Li <min.li.xe@renesas.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>