regulator: rpm-smd-regulator: Add support for proxy consumers

Add support to enforce proxy consumer requests defined in device
tree for rpm-smd-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: I25569808b2026f800f3a43124cf1bf2d8b58c14b
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>