clk: qcom: gpucc-sc8280xp: Add external supply for GX gdsc

clk: qcom: gpucc-sc8280xp: Add external supply for GX gdsc

On SA8295P and SA8540P the GFX rail is powered by a dedicated external
regulator, instead of the rpmh-controlled "gfx.lvl".

Define the "vdd-gfx" as the supply regulator for the GDSC, to cause the
gdsc logic to look for, and control, this external power supply.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Link: https://lore.kernel.org/r/20240125-sa8295p-gpu-v4-3-7011c2a63037@quicinc.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>