PM / Domains: Rename genpd internals from pm_genpd_* to genpd_*

Most of the functions names has already moved the genpd naming rules,
however let's make this complete to avoid any further confusions.

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