ptp: ocp: Add sysfs attribute utc_tai_offset

IRIG and DCF output time in UTC, but the timecard operates
on TAI internally.  Add an attribute node which allows adding
an offset to these modes before output.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>