net: dsa: mv88e6xxx: Abstract PTP operations

The mv88e6165 family supports PTP, but its registers use a different
layout to the currently supported devices. Abstract accessing the PTP
registers into a set of ops, so making space for a second
implementation.

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