ptp: Add a command line option in testptp to set a specific PTP time

Signed-off-by: Manfred Rudigier <manfred.rudigier@omicron.at>
Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>