msm: spm: Add cross-calling support to change voltages

SPM block is responsible for power collapsing the core, but also
interacts with the PMIC through hardware to change core voltage. It
would be beneficial to frequency algorithms to change frequencies and
thereby voltages of all cores running from a single core. In some
targets, the SPM is configured to set voltage only for that core. Ensure
that the core voltage is set only from that core.

Change-Id: I656e20a49d9784f08e9ce4d4e8f6c15760ba015b
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>