dt-bindings: drm/msm/a6xx: Add GX power-domain for GMU bindings

The GMU should have two power domains defined: "cx" and "gx". "cx" is the
actual power domain for the device and "gx" will be attached at runtime
to manage reference counting on the GPU device in case of a GMU crash.

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