ptp: ocp: add tod_correction attribute

TOD correction register is used to compensate for leap seconds in
different domains.  Export it as an attribute with write access.

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>