dt-bindings: net: dsa: qca, ar9331 switch documentation

Atheros AR9331 has built-in 5 port switch. The switch can be configured
to use all 5 or 4 ports. One of built-in PHYs can be used by first built-in
ethernet controller or to be used directly by the switch over second ethernet
controller.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>