net: ptp: introduce common defines for PTP message types

Using PTP wide defines will obsolete different driver internal defines
and uses of magic numbers.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Cc: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>