cpufreq-dt: Handle OPP voltage adjust events

On some SoCs the Adaptive Voltage Scaling (AVS) technique is
employed to optimize the operating voltage of a device. At a
given frequency, the hardware monitors dynamic factors and either
makes a suggestion for how much to adjust a voltage for the
current frequency, or it automatically adjusts the voltage
without software intervention.

In the former case, an AVS driver will call
dev_pm_opp_modify_voltage() and update the voltage for the
particular OPP the CPUs are using. Add an OPP notifier to
cpufreq-dt so that we can adjust the voltage of the CPU when AVS
updates the OPP.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
[georgi.djakov@linaro.org: drop RCU for OPPs]
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>