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 <[email protected]>
Reviewed-by: Konrad Dybcio <[email protected]>
Signed-off-by: Bjorn Andersson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Bjorn Andersson <[email protected]>