ptp: ocp: Add .getmaxphase ptp_clock_info callback

Add a function that advertises a maximum offset of zero supported by
ptp_clock_info .adjphase in the OCP null ptp implementation.

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>
Cc: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Acked-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>