msm: acpuclock-8974: Vote on the PM8941_S2 regulator for HFPLLs

PM8941_S2 is the parent of PM8941_L12, which supplies the HFPLLs.
Because the rpm_regulator_*() APIs do not manage this relationship,
it is necessary to enable both from acpuclock when the HFPLLs are
needed.

Change-Id: I1f60934f1cdfc7acfdf3dec5a793754896c8d33f
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>