dt-bindings: drm/msm/a6xx: Document GMU bindings

Commit 24937c540917 ("dt-bindings: drm/msm/a6xx: Document GMU and update
GPU bindings") mistakenly omitted the GMU bindings as seen in [1].
Return them to their rightful place.

[1] https://patchwork.freedesktop.org/patch/268679/



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