dt-bindings: clock: sm6375-gpucc: Add VDD_GX

The GPUCC block on SM6375 is powered by VDD_CX and VDD_GX. If the latter
rail is not online, GX_GDSC will never turn on. Describe the missing
handles.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230529-topic-sm6375gpuccpd-v1-1-8d57c41a6066@linaro.org