net: dsa: mv88e6xxx: add PPU operations

Some Marvell chips can enable/disable the PPU on demand. This is needed
to access the PHY registers when there is no indirection mechanism.

Add two new ppu_enable and ppu_disable ops to describe this and finally
get rid of the MV88E6XXX_FLAG_PPU* flags.

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