PM / domains: Remove system PM callbacks from gpd_dev_ops

There no users of these callbacks, let's simplify the generic power
domain by removing them.

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>