net: dsa: sja1105: Move PTP data to its own private structure

This is a non-functional change with 2 goals (both for the case when
CONFIG_NET_DSA_SJA1105_PTP is not enabled):

- Reduce the size of the sja1105_private structure.
- Make the PTP code more self-contained.

Leaving priv->ptp_data.lock to be initialized in sja1105_main.c is not a
leftover: it will be used in a future patch "net: dsa: sja1105: Restore
PTP time after switch reset".

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
