msm-core: Use cpu device to get the voltage plan

Clock driver will populate the voltages for the cpu device instead of
the regulator device.

Use the cpu device to get the voltage plan instead of the regulator
device.

Change-Id: I25a5f8bfff6eb28eb815cd7667e53fa0389ab1b5
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>