net: dsa: mv88e6xxx: rework Global2 SMI PHY access

Describe the presence of the Global2 SMI PHY registers, used to
indirectly access the internal SMI devices registers on some chips.

Also temporarily forward declare mv88e6xxx_g2_smi_phy_{read,write} to
use them in mv88e6xxx_mdio_{read,write}_indirect, before getting rid of
the later.

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