net: dsa: mv88e6xxx: prefix Global VTU macros

Prefix and document the Global VTU registers macros and give a clear
16-bit registers representation.

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