dt-bindings: drm/msm/a6xx: Document GMU and update GPU bindings

Update the GPU bindings and document the new bindings for the GMU
device found with Adreno a6xx targets.

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