net: dsa: mv88e6xxx: abstract REG_GLOBAL2

Similarly to the ports, phys, and Global SMI devices, abstract the SMI
device address of the Global 2 registers in a few g2 static helpers.

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