A 4th timestamper is added which timestamps the output of the PHC.

The clock nanosecond offset is not always zero, so when compared
to other timestampers, this provides precise measurements.

Also, the timestamper interrupt from the PHC can be used to generate
a PPS signal for /dev/pps.

Also allow PTP_CLK_REQ_PEROUT requests for a 1PPS output, but do
not actually configure any output pins, this is done via sysfs.
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>