net: dsa: mv88e6xxx: make global2 code optional

Since not every chip has a Global2 set of registers, make its support
optional, in which case the related functions will return -EOPNOTSUPP.

This also allows to reduce the size of the mv88e6xxx driver for devices
such as home routers embedding Ethernet chips without Global2 support.

It is present on most recent chips, thus enable its support by default.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>