regulator: core: add debugfs regulator monitoring and control features

Add several debugfs files for each registered regulator which
allow for monitoring of the regulator state and the requests from
each consumer.  Also configure these files as writeable so that
the state of each regulator can be voted on via debugfs file
write commands.

Change-Id: Ia13b8f698868374499bb9be390ade4519c517e29
Signed-off-by: David Collins <collinsd@codeaurora.org>