drm/msm: add headless gpu device for imx5

This patch allows using drm/msm without qcom display hardware. It adds a
amd,imageon compatible, which is used instead of qcom,adreno, but does
not require a top level msm node.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Rob Clark <robdclark@gmail.com>