regulator: gpio-regulator: create a regulator driver that wraps a GPIO

Create a new regulator driver, gpio-regulator, which wraps around a
GPIO that is the enable signal for a physical regulator.

Change-Id: I541a9a1cd3a6ffe3200ffaeee9f0f62db5e65795
Signed-off-by: David Collins <collinsd@codeaurora.org>
[sboyd: Moved to msm-gpio-regulator to not conflcit with
upstream's version of essentially the same driver]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>