Similarly to the VTU, PVT and ATU setup, provide a mv88e6xxx_phy_setup
helper which wraps mv88e6xxx_ppu_enable, so that no more PPU-related
functions are exposed outside of phy.c.

Thus make mv88e6xxx_ppu_enable static.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>