net: fec: Add support for periodic output signal of PPS

This patch adds the support for configuring periodic output
signal of PPS. So the PPS can be output at a specified time
and period.
For developers or testers, they can use the command "echo
<channel> <start.sec> <start.nsec> <period.sec> <period.
nsec> > /sys/class/ptp/ptp0/period" to specify time and
period to output PPS signal.
Notice that, the channel can only be set to 0. In addtion,
the start time must larger than the current PTP clock time.
So users can use the command "phc_ctl /dev/ptp0 -- get" to
get the current PTP clock time before.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>