regulator: add refgen regulator driver

Some Qualcomm Technologies, Inc. SoCs such as SDM845 utilize
reference bias generators for various internal PHY blocks.  These
are called REFGENs.  Add a driver to support REFGEN so that
consumers can vote based upon their runtime hardware needs.

Change-Id: I60681273cc1f9842844968bf4f0f682cdd28eb0d
Signed-off-by: David Collins <collinsd@codeaurora.org>