regulator: add stub-regulator driver

Add a stub-regulator driver which can be used to simulate any
particular regulator.  This stub regulator driver must support
regulator_set_voltage() which is not possible via the existing
dummy regulator.  Stub regulator devices can be used to verify
that regulator consumers make correct regulator framework calls
before it is possible to implement true regulator control.

Change-Id: Ia73d245419e44d74712b813e58de1973112233c4
Signed-off-by: David Collins <collinsd@codeaurora.org>