Prefix and document the Global Switch MAC Address Register macros and
give clear 16-bit register representation.

At the same time, move mv88e6xxx_g1_set_switch_mac in global1.c, where
it belongs.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>