msm: copper: Enable stub regulator driver

stub-regulator is used to support the standard regulator
interfaces until the hardware-based implementation is ready.
Client calls behave just like the a real driver, except the
APIs have no side effect.

Change-Id: I2ccd6ccc5e582743821cbffdfc576122d3252368
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>