regulator: Add stub regulator support

Add support for a dummy driver that has an implementation
connected to no real hardware devices. Its purpose is to aid
bringup scenarios by providing clients a regulator interface
that they expect. This driver shouldn't be enabled if a real
device implementation for a particular platform already exists.

Change-Id: Ib83027b4e599ebbc3cb153b2f346bb03495bb746
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>