dt-bindings: display/msm/gmu: Add GMU wrapper

The "GMU Wrapper" is Qualcomm's name for "let's treat the GPU blocks
we'd normally assign to the GMU as if they were a part of the GMU, even
though they are not". It's a (good) software representation of the GMU_CX
and GMU_GX register spaces within the GPUSS that helps us programatically
treat these de-facto GMU-less parts in a way that's very similar to their
GMU-equipped cousins, massively saving up on code duplication.

The "wrapper" register space was specifically designed to mimic the layout
of a real GMU, though it rather obviously does not have the M3 core et al.

To sum it all up, the GMU wrapper is essentially a register space within
the GPU, which Linux sees as a dumbed-down regular GMU: there's no clocks,
interrupts, multiple reg spaces, iommus and OPP. Document it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542750/


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