net: dsa: mv88e6xxx: factorize PHY access with PPU

Add a MV88E6XXX_FLAG_PPU flag to describe switch models with a PHY
Polling Unit. This allows to merge PPU specific PHY access code in the
share code.

Make the mv88e6xxx_ppu_disable and mv88e6xxx_phy_{read,write}_ppu
functions use unlocked register accesses in order to call them in
mv88e6xxx_phy_{read,write} in a locked context.

Since the PPU code is shared, also remove NET_DSA_MV88E6XXX_NEED_PPU.

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