net: dsa: mv88e6xxx: Add MV88E6165 AVB register access

The MV88E6165 PTP registers are all in AVB bank F, unlike newer
generations which spread them over AVB bank E and F. Implement AVB ops
for the MV88E6165 which hides this difference.

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