msm: rpm-regulator-smd: Add support for software control mode configuration

Add an API, rpm_regulator_set_mode(), to the rpm-regulator-smd
driver which can be used to configure which software control mode
to use for RPM regulators.  One important control mode that can
be selected via this API is AUTO mode where the SMPS regulator
hardware automatically switches between PFM and PWM mode based
upon realtime load.  This mode configuration doesn't map into the
regulator framework so a special purpose API is needed.

Change-Id: I3a648d91895e8dbce8551708e931148fab50bab6
Signed-off-by: David Collins <collinsd@codeaurora.org>