msm: rpm-regulator: allow disabling via rpm_vreg_set_voltage

Provide a means for consumers to vote to disable a regulator
using the rpm_vreg_set_voltage API.  Change the constraint
checking so that min_uV = max_uV = 0 becomes a disable request.

Change-Id: Ibaa8b862e9591228dbb9d17eaf61d91e07574069
Signed-off-by: David Collins <collinsd@codeaurora.org>