OPP: Rework _set_required_devs() to manage a single device per call

OPP: Rework _set_required_devs() to manage a single device per call

At this point there are no consumer drivers that makes use of
_set_required_devs(), hence it should be straightforward to rework the code
to enable it to better integrate with the PM domain attach procedure.

During attach, one device at the time is being hooked up to its
corresponding PM domain. Therefore, let's update the _set_required_devs()
to align to this behaviour, allowing callers to fill out one required_dev
per call. Subsequent changes starts making use of this.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20241002122232.194245-4-ulf.hansson@linaro.org