regulator: add proxy consumer library

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

Change-Id: I46853f55a1201195ed50f0365399cf49086f24ff
Signed-off-by: David Collins <collinsd@codeaurora.org>