dt-bindings: display/msm/rgmu: Document A612 RGMU

dt-bindings: display/msm/rgmu: Document A612 RGMU

RGMU a.k.a Reduced Graphics Management Unit is a small state machine
with the sole purpose of providing IFPC (Inter Frame Power Collapse)
support. Compared to GMU, it doesn't manage GPU clock, voltage
scaling, bw voting or any other functionalities. All it does is detect
an idle GPU and toggle the GDSC switch. As it doesn't access DDR space,
it doesn't require iommu.

So far, only Adreno 612 GPU has an RGMU core. Document it in
qcom,adreno-rgmu.yaml.

Signed-off-by: Jie Zhang <jie.zhang@oss.qualcomm.com>
Signed-off-by: Akhil P Oommen <akhilpo@oss.qualcomm.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Patchwork: https://patchwork.freedesktop.org/patch/696679/
Message-ID: <20251231-qcs615-spin-2-v6-4-da87debf6883@oss.qualcomm.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>