ptp: Add generic PTP is_sync() function

[ Upstream commit f72de02e ]

PHY drivers such as micrel or dp83640 need to analyze whether a given
skb is a PTP sync message for one step functionality.

In order to avoid code duplication introduce a generic function and
move it to ptp classify.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 9b2bfdbf ("phy: mscc: Stop taking ts_lock for tx_queue and use its own lock")
Signed-off-by: Sasha Levin <sashal@kernel.org>