PM / OPP: Reword binding supporting multiple regulators per device

PM / OPP: Reword binding supporting multiple regulators per device

On certain platforms (like TI), DVFS for a single device (CPU) requires
configuring multiple power supplies.

The OPP bindings already contains binding and example to explain this
case, but it isn't sufficient.

- There is no way for the code parsing these bindings to know which
  voltage values belong to which power supply.

- It is not possible to know the order in which the supplies need to be
  configured while switching OPPs.

This patch clarifies on those details by mentioning that such
information is left for the implementation specific bindings to explain.
They may want to hardcode such details or implement their own properties
to get such information. All implementations using multiple regulators
for their devices must provide a binding document explaining their
implementation.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>