PM / domains: Remove redundant check for CONFIG_PM

CONFIG_PM_GENERIC_DOMAINS depends on CONFIG_PM, thus there are no need
to check explicity for it.

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