dt-bindings: display/msm: qcom, sm8250-mdss: add DisplayPort controller node

Document the DisplayPort controller node in MDSS binding, already used
in DTS:

sm8250-xiaomi-elish-boe.dtb: display-subsystem@ae00000: Unevaluated properties are not allowed ('displayport-controller@ae90000' was unexpected)

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/566297/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Baryshkov <[e-mail]>

Documentation/devicetree/bindings/display/msm/qcom,sm8250-mdss.yaml

···
       compatible:
         const: qcom,sm8250-dpu
 
+  "^displayport-controller@[0-9a-f]+$":
+    type: object
+    additionalProperties: true
+
+    properties:
+      compatible:
+        items:
+          - const: qcom,sm8250-dp
+          - const: qcom,sm8350-dp
+
   "^dsi@[0-9a-f]+$":
     type: object
     additionalProperties: true