net: dsa: mv88e6352: Refactor shareable code

The mv88e6352 allows access to the port phys via an internal mdio bus
which is accessed using registers in the GLOBAL 2 range. The mv88e6171
and probably other devices use the same mechanism. Move this code into
the shared mv88e6xxx.c library.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>