net: dsa: mv88e6xxx: add PTP support for MV88E6250 family

This adds PTP support for the MV88E6250 family.

Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>