dt-bindings: Fix 'unevaluatedProperties' errors in DT graph users

In testing out under development json-schema 2020-12 support, there's a
few issues with 'unevaluatedProperties' and the graph schema. If
'graph.yaml#/properties/port' is used, then neither the port nor the
endpoint(s) can have additional properties. 'graph.yaml#/$defs/port-base'
needs to be used instead.

Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: "Paul J. Murphy" <paul.j.murphy@intel.com>
Cc: Daniele Alessandrelli <daniele.alessandrelli@intel.com>
Cc: "Niklas Söderlund" <niklas.soderlund@ragnatech.se>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linux-renesas-soc@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210623164344.2571043-1-robh@kernel.org

Documentation/devicetree/bindings/display/bridge/lontium,lt8912b.yaml

@@ -29,7 +29,8 @@ properties:
 
     properties:
       port@0:
-        $ref: /schemas/graph.yaml#/properties/port
+        $ref: /schemas/graph.yaml#/$defs/port-base
+        unevaluatedProperties: false
         description:
           Primary MIPI port for MIPI input