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.

Change-Id: Ib76c789cdf14f8906e6eeb026cf6117f1f8a8e20
CRs-Fixed: 1082230
Signed-off-by: David Collins <collinsd@codeaurora.org>