msm: rpm-regulator: Support rpm_regulator_* API in rpm-regulator driver

Modify the rpm-regulator driver so that it can support the API
defined by the rpm-regulator-smd driver:

rpm_regulator_get
rpm_regulator_put
rpm_regulator_enable
rpm_regulator_disable
rpm_regulator_set_voltage

Change-Id: Ib0b76e96c3d5dcaa4ab993751cea11c0d51fc4af
Signed-off-by: David Collins <collinsd@codeaurora.org>