arm64: dts: qcom: sm6350: Replace literal rpmhpd indices with constants

It seems the SM6350_CX definition was temporarily replaced with its
literal value 0 in 1797e1c9 ("arm64: dts: qcom: sm6350: Add SDHCI1/2
nodes") to prevent a dependency on the qcom-rpmpd.h header patch being
available prior to this DT patch being applied, similar to c23f1b77
("arm64: dts: qcom: sm6125: Avoid using missing SM6125_VDDCX").
However, unlike the revert of that in the sm6125 tree the next merge
window around in a90b8adf ("Revert "arm64: dts: qcom: sm6125: Avoid
using missing SM6125_VDDCX""), this has not yet happened for sm6350:
replace them back now that the definitions are definitely available.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220507224645.2238421-1-marijn.suijten@somainline.org