net: dsa: mv88e6xxx: Add hwtimestamp support for the 6165

The 6165 family supports a more restricted version of hardware time
stamps. Only L2 PTP is supported. All ports have to use the same
EtherType, and transport spec configuration. PTP can only be
enabled/disabled globally, not per port.

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