Revert "arm64: dts: qcom: sm6125: Avoid using missing SM6125_VDDCX"

This reverts commit c23f1b77.

The SM6125_VDDCX constant was replaced with 0 temporarily as the header
patch defining this constant resided in a different branch, creating an
unwanted dependency of the dts branch on the drivers branch.
Now (by the time this patch will be applied) that both branches have
been merged upstream, it is safe to revert to the constant again.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211229220117.293542-1-marijn.suijten@somainline.org