msm: spm-regulator: add support for specifying CPU mapping

Add support for a new optional device tree property,
qcom,cpu-num, which can be used to specify which CPU a given SPM
regulator device powers.  This mapping is needed for boards where
each SPM regulator device powers a single CPU core.

Change-Id: I3b064a50dd7568f40494e9b2c8edcf5525bc8d39
Signed-off-by: David Collins <collinsd@codeaurora.org>