regulator: refgen: add support for proxy consumers

Add support to enforce proxy consumer requests defined in device
tree for refgen devices.  These can be used to ensure that
regulators are not accidentally disabled or scaled down
before all consumers have had a chance to make their own
requests.

Change-Id: I57ce2ac4162c29e749e1cd915241593d4ad262dc
Signed-off-by: David Collins <collinsd@codeaurora.org>