dt-bindings: tda998x: Document the required 'port' node.

All the users of the tda998x driver are component based and bind the
driver via the device graph method described in
Documentation/devicetree/bindings/graph.txt. Add the fact that the
'port' node is required to the bindings.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>