The generic PM domain is currently the only PM domain variant that supports
performance scaling. To allow performance scaling to be supported through a
common interface, let's add an optional callback ->set_performance_state(),
in the struct dev_pm_domain.

Moreover, let's add a function, dev_pm_domain_set_performance_state(), that
may be called by consumers to request a new performance state for a device
through its PM domain.

Note that, in most cases it's preferred that a consumer use the OPP library
to request a new performance state for its device. Although, this requires
some additional changes to be supported, which are being implemented from
subsequent changes.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>