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>

Documentation/devicetree/bindings/display/bridge/tda998x.txt

@@ -5,6 +5,10 @@ Required properties;
 
   - reg: I2C address
 
+Required node:
+  - port: Input port node with endpoint definition, as described
+        in Documentation/devicetree/bindings/graph.txt
+
 Optional properties:
   - interrupts: interrupt number and trigger type
 	default: polling