net: dsa: mv88e6xxx: move the Global 2 macros

Move the GLOBAL2_* macros where they belong, in the related global2.h
header.

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