This enables the PTP support towards userspace applications such as
linuxptp.

The switches can timestamp only trapped multicast MAC frames, and
therefore only the profiles of 1588 over L2 are supported.

TX timestamping can be enabled per port, but RX timestamping is enabled
globally. As long as RX timestamping is enabled, the switch will emit
metadata follow-up frames that will be processed by the tagger. It may
be a problem that linuxptp does not restore the RX timestamping settings
when exiting.

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