net: dsa: add support for Atheros AR9331 built-in switch

Provide basic support for Atheros AR9331 built-in switch. So far it
works as port multiplexer without any hardware offloading support.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>