drm/tilcdc: Refer to panel.txt and tfp410.txt bindings in tilcdc.txt

The legacy panel.txt and tfp410.txt bindings are still the only supported
way to connect lcd panel and tfp410 DVI encoder to tilcdc.
Signed-off-by: NJyri Sarha <jsarha@ti.com>

Documentation/devicetree/bindings/display/tilcdc/tilcdc.txt

@@ -24,6 +24,10 @@ Optional nodes:
    binding follows Documentation/devicetree/bindings/graph.txt and
    suppors a single port with a single endpoint.
 
+ - See also Documentation/devicetree/bindings/display/tilcdc/panel.txt and
+   Documentation/devicetree/bindings/display/tilcdc/tfp410.txt for connecting
+   tfp410 DVI encoder or lcd panel to lcdc
+
 Example:
 
 	fb: fb@4830e000 {