net: dsa: mv88e6xxx: Abstract supported PTP filters

The 6165 only supports layer L2 PTP, where as the more modern devices
also support UDP and UDPv6, i.e. L4. Abstract the supported receive
filters.

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