net: dsa: mv88e6xxx: add a soft reset operation

Marvell chips have different way to issue a software reset.

Old chips (such as 88E6060) have a reset bit in an ATU control register.

Newer chips moved this bit in a Global control register. Chips with
controllable PPU should reset the PPU when resetting the switch.

Add a new reset operation to implement these differences and introduce a
mv88e6xxx_software_reset() helper to wrap it conveniently.

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