net: dsa: sja1105: Build PTP support in main DSA driver

As Arnd Bergmann pointed out in commit 78fe8a28 ("net: dsa: sja1105:
fix ptp link error"), there is no point in having PTP support as a
separate loadable kernel module.

So remove the exported symbols and make sja1105.ko contain PTP support
or not based on CONFIG_NET_DSA_SJA1105_PTP.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>