dt-bindings: display: bridge: tc358867: Document default DP preemphasis

Document default DP port preemphasis configurable via new DT property
"toshiba,pre-emphasis". This is useful in case the DP link properties
are known and starting link training from preemphasis setting of 0 dB
is not useful. The preemphasis can be set separately for both DP lanes
in range 0=0dB, 1=3.5dB, 2=6dB .

This is an endpoint property, not a port property, because the TC9595
datasheet does mention that the DP might operate in some sort of split
mode, where each DP lane is used to feed one display, so in that case
there might be two endpoints.
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708150130.54484-1-marex@denx.de

Documentation/devicetree/bindings/display/bridge/toshiba,tc358767.yaml

@@ -92,12 +92,31 @@ properties:
             reference to a valid DPI output or input endpoint node.
 
       port@2:
-        $ref: /schemas/graph.yaml#/properties/port
+        $ref: /schemas/graph.yaml#/$defs/port-base
+        unevaluatedProperties: false
         description: |
             eDP/DP output port. The remote endpoint phandle should be a
             reference to a valid eDP panel input endpoint node. This port is
             optional, treated as DP panel if not defined
 
+        properties:
+          endpoint:
+            $ref: /schemas/media/video-interfaces.yaml#
+            unevaluatedProperties: false
+
+            properties:
+              toshiba,pre-emphasis:
+                description:
+                  Display port output Pre-Emphasis settings for both DP lanes.
+                $ref: /schemas/types.yaml#/definitions/uint8-array
+                minItems: 2
+                maxItems: 2
+                items:
+                  enum:
+                    - 0 # No pre-emphasis
+                    - 1 # 3.5dB pre-emphasis
+                    - 2 # 6dB pre-emphasis
+
     oneOf:
       - required:
           - port@0