PM / Domains: Drop unused parameter in genpd_allocate_dev_data()

The in-parameter struct generic_pm_domain *genpd to
genpd_allocate_dev_data() is unused, so let's drop it.

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