dt-bindings: net: dsa: qca8k: improve port definition documentation

Clean and improve port definition for qca8k documentation by referencing
the dsa generic port definition and adding the additional specific port
definition.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>