regulator: add QCOM RPMh regulator driver

Add the QCOM RPMh regulator driver to manage PMIC regulators
which are controlled via RPMh on some Qualcomm Technologies, Inc.
SoCs.  RPMh is a hardware block which contains several
accelerators which are used to manage various hardware resources
that are shared between the processors of the SoC.  The final
hardware state of a regulator is determined within RPMh by
performing max aggregation of the requests made by all of the
processors.

Add support for PMIC regulator control via the voltage regulator
manager (VRM) and oscillator buffer (XOB) RPMh accelerators.
VRM supports manipulation of enable state, voltage, and mode.
XOB supports manipulation of enable state.

Signed-off-by: David Collins <collinsd@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>