pmdomain: core: Manage the default required OPP from a separate function

To improve the readability of the code in __genpd_dev_pm_attach(), let's
move out the required OPP handling into a separate function.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20241002122232.194245-6-ulf.hansson@linaro.org