PM: domains: create debugfs nodes when adding power domains

debugfs nodes were created in genpd_debug_init alled in late_initcall
preventing power domains registered though loadable modules to have
a debugfs entry.

Create/remove debugfs nodes when the power domain is added/removed
to/from the internal gpd_list.

Signed-off-by: Thierry Strudel <tstrudel@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>