ptp: introduce get/set time methods with explicit 64 bit seconds.

Converting the PHC drivers over to the new methods is one step along the
way to making them ready for 2038.  Once all the drivers are up to date,
then the old methods will be removed.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>