net: dsa: mv88e6xxx: link mv88e6xxx_port to mv88e6xxx_chip

An up coming change will register interrupts for individual switch
ports, using the mv88e6xxx_port as the interrupt context information.
Add members to the mv88e6xxx_port structure so we can link it back to
the mv88e6xxx_chip member the port belongs to and the port number of
the port.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>