dt-bindings: display: msm/gpu: document amd,imageon compatible

Document the new amd,imageon compatible, used for non-qcom hardware that
uses the drm/msm driver (iMX5).
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

Documentation/devicetree/bindings/display/msm/gpu.txt

 Qualcomm adreno/snapdragon GPU
 
 Required properties:
-- compatible: "qcom,adreno-XYZ.W", "qcom,adreno"
+- compatible: "qcom,adreno-XYZ.W", "qcom,adreno" or
+	      "amd,imageon-XYZ.W", "amd,imageon"
     for example: "qcom,adreno-306.0", "qcom,adreno"
   Note that you need to list the less specific "qcom,adreno" (since this
   is what the device is matched on), in addition to the more specific
   with the chip-id.
+  If "amd,imageon" is used, there should be no top level msm device.
 - reg: Physical base address and length of the controller's registers.
 - interrupts: The interrupt signal from the gpu.
 - clocks: device clocks