dt-bindings: net: dsa: mt7530: Add support for port 5

MT7530 port 5 has many modes/configurations.
Update the documentation how to use port 5.

Signed-off-by: René van Dorst <opensource@vdorst.com>
Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>