net: ptp: add a definition for the UDP port for IEEE 1588 general messages

As opposed to event messages (Sync, PdelayReq etc) which require
timestamping, general messages (Announce, FollowUp etc) do not.
In PTP they are part of different streams of data.

IEEE 1588-2008 Annex D.2 "UDP port numbers" states that the UDP
destination port assigned by IANA is 319 for event messages, and 320 for
general messages. Yet the kernel seems to be missing the definition for
general messages. This patch adds it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>