dt-bindings: net: dsa: qca8k: Document support for CPU port 6

The switch now support CPU port to be set 6 instead of be hardcoded to
0. Document support for it and describe logic selection.
Signed-off-by: NAnsuel Smith <ansuelsmth@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/qca8k.txt

@@ -29,7 +29,11 @@ the mdio MASTER is used as communication.
 Don't use mixed external and internal mdio-bus configurations, as this is
 not supported by the hardware.
 
-The CPU port of this switch is always port 0.
+This switch support 2 CPU port. Normally and advised configuration is with
+CPU port set to port 0. It is also possible to set the CPU port to port 6
+if the device requires it. The driver will configure the switch to the defined
+port. With both CPU port declared the first CPU port is selected as primary
+and the secondary CPU ignored.
 
 A CPU port node has the following optional node: