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>

include/linux/ptp_classify.h

@@ -37,6 +37,7 @@
 #define PTP_MSGTYPE_PDELAY_RESP 0x3
 
 #define PTP_EV_PORT 319
+#define PTP_GEN_PORT 320
 #define PTP_GEN_BIT 0x08 /* indicates general message, if set in message type */
 
 #define OFF_PTP_SOURCE_UUID	22 /* PTPv1 only */