net: dsa: mv88e6xxx: describe PHY page and SerDes

Add mv88e6xxx_phy_page_{read,write} routines and use them to access the
SerDes PHY device registers.

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