pmdomain: core: Set the required dev for a required OPP during genpd attach

In the single PM domain case there is no need for platform code to specify
the index of the corresponding required OPP in DT, as the index must be
zero. This allows us to assign a required dev for the required OPP from
genpd, while attaching a device to its PM domain.

In this way, we can remove some of the genpd specific code in the OPP core
for the single PM domain case. Although, this cleanup is made from a
subsequent change.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20241002122232.194245-7-ulf.hansson@linaro.org