msm: board-msm8960: switch to RPM control for most regulators

Switch control of most regulators from the pm8921-regulator
driver to the rpm-regulator driver.  pm8921-regulator will still
be used for USB_OTG and HDMI_MVS.  pm8921-regulator will also
remain in use for LDO 26, 27, 28, and 29 because these are not
handled by the RPM. saw-regulator will continue to be used for
8921_s5 and 8921_s6 (Krait rail supplies).

Also un-stub functions rpm_vreg_set_voltage and
rpm_vreg_set_frequency now that they can safely be called.

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