ptp: add getcrosststamp() to virtual clocks.

If the physical clock supports cross timestamping (it has the
getcrosststamp() function), provide a wrapper in the virtual clock to
enable cross timestamping.

This adds support for the PTP_SYS_OFFSET_PRECISE ioctl.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>