net: dsa: hellcreek: Allow PTP P2P measurements on blocked ports

Allow PTP peer delay measurements on blocked ports by STP. In case of topology
changes the PTP stack can directly start with the correct delays.

Fixes: ddd56dfe ("net: dsa: hellcreek: Add PTP clock support")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>