drm/msm/a6xx: Rename gmu phandle to qcom,gmu

>From the review for the DT bindings for the GPU/GMU it
was suggested that the phandle for the GMU be
'qcom,gmu' instead of just 'gmu'.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>