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: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>