ptp: ocp: Expose clock status drift and offset

Monitoring of clock variance could be done through checking
the offset and the drift updates that are applied to atomic
clocks.  Expose these values as attributes for the timecard.

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