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>