To extend genpd to deal with allowing multiple PM domains per device, some
of the code in genpd_dev_pm_attach() can be re-used. Let's prepare for this
by moving some of the code into a sub-function.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org> Acked-by: Jon Hunter <jonathanh@nvidia.com> Tested-by: Jon Hunter <jonathanh@nvidia.com> Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>