PM / Domains: Align in-parameter names for some genpd functions

Align in-parameter names for the declarations of pm_genpd_add|
remove_subdomain() and of_genpd_add_subdomain() according to their
implementations, as to improve consistency.

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