net: Introduce peer to peer one step PTP time stamping.

The 1588 standard defines one step operation for both Sync and
PDelay_Resp messages.  Up until now, hardware with P2P one step has
been rare, and kernel support was lacking.  This patch adds support of
the mode in anticipation of new hardware developments.

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