net: dsa: mv88e6xxx: Add mv88e6165 PTP support

The mv88e6165 family has its global clock in the PTP global
registers. It does not support any form of PTP events. Add a function
to read the clock, fill in an ops structure, and register it with the
two members of the family.

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