dt-bindings: regulator: add rpmh-regulator bindings

Add rpmh-regulator device bindings for PMIC regulator management
via the RPMh hardware block on Qualcomm Technologies, Inc. SoCs.

This is a snapshot commit of msm-4.14 as of:
commit 2913cfe33e10 ("Merge "iommu: arm-smmu: wait for writes to
complete before power off"").  Some modifications have been made
to handle rpmh device binding differences between msm-4.14 and
msm-4.19.

Change-Id: I466cfea65acc11fd54c3b292c4d21cbac6f9cf40
Signed-off-by: David Collins <collinsd@codeaurora.org>