pps: parport: use timespec64 instead of timespec

getnstimeofday() is deprecated, so I'm converting this to use
ktime_get_real_ts64() as a safe replacement.  I considered using
ktime_get_real() instead, but since the algorithm here depends on the
exact timing, I decided to introduce fewer changes and leave the code
that determines the nanoseconds since the last seconds wrap untouched.

It's not entirely clear to me whether we should also change the time
base to CLOCK_BOOTTIME or CLOCK_TAI.  With boottime, we would be
independent of changes due to settimeofday() and only see the speed
adjustment from the upstream clock source, with the downside of having
the signal be at an arbirary offset from the start of the UTC second
signal.  With CLOCK_TAI, we would use the same offset from the UTC
second as before and still suffer from settimeofday() adjustments, but
would be less confused during leap seconds.

Both boottime and tai only offer usable (i.e.  avoiding ktime_t to
timespec64 conversion) interfaces for kt...