ANDROID: GKI: 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.

Bug: 150508586
Change-Id: I0ccc63a41684462684ac737fb2f4129a3e6e4aea
Signed-off-by: David Collins <collinsd@codeaurora.org>
(cherry picked from commit cf03466c)
[Also squashed e9a05bb4 "spdx: Modify spdx tag from GPL-2.0 to GPL-2.0-only"]
Signed-off-by: Saravana Kannan <saravanak@google.com>