net: mscc: ocelot: support PTP Sync one-step timestamping

Although HWTSTAMP_TX_ONESTEP_SYNC existed in ioctl for hardware timestamp
configuration, the PTP Sync one-step timestamping had never been supported.

This patch is to truely support it.

- ocelot_port_txtstamp_request()
  This function handles tx timestamp request by storing
  ptp_cmd(tx timestamp type) in OCELOT_SKB_CB(skb)->ptp_cmd,
  and additionally for two-step timestamp storing ts_id in
  OCELOT_SKB_CB(clone)->ptp_cmd.

- ocelot_ptp_rew_op()
  During xmit, this function is called to get rew_op (rewriter option) by
  checking skb->cb for tx timestamp request, and configure to transmitting.

Non-onestep-Sync packet with one-step timestamp request falls back to use
two-step timestamp.

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