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