PM / Domains: Provide dummy governors if CONFIG_PM_GENERIC_DOMAINS=n

This allows to compile-test drivers that refer to governors (always by
reference) when CONFIG_PM_GENERIC_DOMAINS=n.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>