ptp: add ptp virtual clock driver framework

This patch is to add ptp virtual clock driver framework
utilizing timecounter/cyclecounter.

The patch just exports two essential APIs for PTP driver.

- ptp_vclock_register()
- ptp_vclock_unregister()

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>