PM / Domains: Drop __pm_genpd_add_device()

There are still a few non-DT existing users of genpd, however neither of
them uses __pm_genpd_add_device(), hence let's drop it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>