net: dsa: mv88e6xxx: prefix Global 2 Trunk macros

Prefix and document the Global 2 Trunk registers macros. At the same
time, fix the hask -> hash typo and use the mv88e6xxx_port_mask helper.

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