PM: domains: fix some kernel-doc issues

Fix the following make W=1 kernel build warnings:

  drivers/base/power/domain_governor.c:259: warning: Function parameter or member 'now' not described in '_default_power_down_ok'
  drivers/base/power/domain.c:581: warning: Function parameter or member 'depth' not described in 'genpd_power_off'
  drivers/base/power/domain.c:2520: warning: Function parameter or member 'np' not described in 'of_genpd_remove_last'
  drivers/base/power/domain.c:2520: warning: Excess function parameter 'provider' description in 'of_genpd_remove_last'

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>