net: dsa: mv88e6xxx: prefix Global Monitor Control macros

Prefix and document the Global Monitor Control Register macros
(which became the Global Monitor & MGMT Control Register with 88E6390)
and give a clear 16-bit registers representation.

Use __bf_shf to get the shift value at compile time instead of adding
new defined macros for it.

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