PM / Domains: Rename argument of pm_genpd_add_subdomain()

Change the name of the second argument of pm_genpd_add_subdomain()
so that it is (a) shorter and (b) in agreement with the name of
the second argument of pm_genpd_add_subdomain().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>