net: dsa: mv88e6xxx: move Global2 code

Marvell chips are composed of multiple SMI devices. One of them at
address 0x1C is called Global2. It provides an extended set of
registers, used for interrupt control, EEPROM access, indirect PHY
access (to bypass the PHY Polling Unit) and cross-chip related setup.

Most chips have it, but some others don't (older ones such as 6060).

Now that its related code is isolated in mv88e6xxx_g2_* functions, move
it to its own global2.c file, making most of its setup code static.
Document each registers in the meantime.

Its compilation can be later avoided for chips without such registers.

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