net: dsa: mv88e6xxx: prefix Global 2 Watchdog macros

The Marvell 88E6352 family has a Global 2 register dedicated to the
watchdog setup. But the 88E6390 turned it into an indirect table.

Prefix and document that.

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