net: dsa/mv88e6352: Add support for MV88E6176

MV88E6176 is mostly compatible to MV88E6352 and is documented
in the same functional specification. Add support for it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>