PM / Domains: Drop unused in-parameter to some genpd functions

Both genpd_alloc_dev_data() and genpd_add_device(), that are internal
genpd functions, allow a struct gpd_timing_data *td to be passed as an
in-parameter. However, as NULL is always passed, let's just drop the
in-parameter altogether.

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