msm: rpm-regulator: Add stubs for rpm_vreg_set_*() APIs on copper

Copper platform clock and acpuclock drivers need to call the
rpm_vreg_set_vdd() API to set vdd_dig and vdd_mem voltages. Until a
full rpm-regulator driver for copper is available, add stub functions
that always return success.

To do this a new MSM_RPM_REGULATOR Kconfig is added.

Change-Id: I0e7c3ab7cd56f7760f0083f4aaac000fdb56d0ec
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>