regulator: rpmh-regulator: add support for proxy consumers

Add support to enforce proxy consumer requests defined in device
tree for rpmh-regulator 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: Ic5f4ef7369584433b0a187f966d2d5f379b98cb6
Signed-off-by: David Collins <collinsd@codeaurora.org>