dt-bindings: opp: Introduce opp-level bindings

Add opp-level as an additional property in the OPP node to describe
the performance level of the device.

On some SoCs (especially from Qualcomm and MediaTek) this value
is communicated to a remote microprocessor by the CPU, which
then takes some actions (like adjusting voltage values across various
rails) based on the value passed.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>