dt-bindings: display/msm/gmu: Document Adreno 750 GMU

Document the Adreno 750 GMU found on the SM8650 platform.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/578684/


Signed-off-by: Rob Clark <robdclark@chromium.org>