regulator: Add proxy consumer driver

Add a proxy consumer driver which can be used to ensure that a
given regulator maintains a certain minimum state during bootup.
Enable state, voltage, and current may be forced to specified
levels.

Change-Id: I0ccc63a41684462684ac737fb2f4129a3e6e4aea
Signed-off-by: David Collins <collinsd@codeaurora.org>