net: dsa: Centralize Marvell switch reset

Marvell switches are all reset in nearly the same way. The only
difference is if the PPU should be enabled or not. Move this
code into the shared mv88x6xxx.c.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>