regulator: add rpmh-regulator driver

Add the rpmh-regulator driver to support PMIC regulator
management via the RPMh hardware block on Qualcomm Technologies,
Inc. SoCs.

rpmh-regulator devices support PMIC regulator management via the
VRM and ARC RPMh accelerators.  The APPS processor communicates
with these hardware blocks via an RSC using command packets.  The
VRM allows changing four parameters for a given regulator: enable
state, output voltage, operating mode, and minimum headroom
voltage.  The ARC allows changing only a single parameter for a
given regulator: its operating level.  This operating level is
fed into CPR which then decides upon a final explicit voltage for
the regulator.

This is a snapshot commit of msm-4.14 as of:
commit 2913cfe3 ("Merge "iommu: arm-smmu: wait for writes to
complete before power off"").  Some modifications have been made
to handle rpmh API differences between msm-4.14 and msm-4.19.

Change-Id: I48e9ab5db808036592cfb26042d843a153fbc92c
Signed-off-by: David Collins <collinsd@codeaurora.org>