regulators: rpm-smd: Remove unused exported functions

Remove all exported functions as clients need to use only
functions exported from core regulator framework.
Change rpm-smd config to tristate to ensure it can be
loaded as a module.

Change-Id: I3f72a19b23fb2b97e0a13fd93fad5eade15ff535
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>