net: dsa: mv88e6xxx: rename PHY PPU accessors

Make it clear that mv88e6xxx_phy_ppu_{read,write} are an implementation
of the .phy_{read,write} operations, by renaming them with the mv88e6185
prefix, since 88E6185 it is the reference switch model supported in an
upstream board (ZII Dev Rev B), which makes use of them.

Distinguish the signatures of implementation specific and generic PHY
functions in the phy.h header.

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