regulator: spm-regulator: add support for bypassing spm APIs

Add support in the spm-regulator driver to configure the PMIC
regulator voltage via SPMI instead of via SPM.  This can be used
when simulated chips do not have fully functional SPM models.

Change-Id: Id75b3dd580ed267dfc5c647d36756690d32b774f
Signed-off-by: David Collins <collinsd@codeaurora.org>