net: dsa: mv88e6xxx: prefix Global Prio and Tag macros

Prefix and document the remaining Global IP and IEEE Priority and Core
Tag Type registers and give them a clear 16-bit register representation.

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