net: dsa: mv88e6xxx: remove unused 88E6165 register definitions

Remove the unused 88E6165 register definitions. For the port
registers, add a comment describing that each arrival and departure
offset is for a set of four registers that correspond with status,
two timestamp registers and the PTP sequence ID captured from the
packet.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>