dt-bindings: display: vc4: dpi: Add missing clock-names property

While the device tree and the driver expected a clock-names property, it
wasn't explicitly documented in the previous binding. Make sure it is now.

Cc: devicetree@vger.kernel.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/8c7a4a9d85f7d1b23346367acf8d321ddad7cffe.1590594512.git-series.maxime@cerno.tech

Documentation/devicetree/bindings/display/brcm,bcm2835-dpi.yaml

@@ -21,6 +21,11 @@ properties:
       - description: The core clock the unit runs on
       - description: The pixel clock that feeds the pixelvalve
 
+  clock-names:
+    items:
+      - const: core
+      - const: pixel
+
   port:
     type: object
     description: >
@@ -31,6 +36,7 @@ required:
   - compatible
   - reg
   - clocks
+  - clock-names
   - port
 
 additionalProperties: false