It is quite common for PM Domains to require slave devices to be kept
active during system suspend if they are to be used as wakeup sources.
To enable this, currently each PM Domain or driver has to provide its
own gpd_dev_ops.active_wakeup() callback.

Introduce a new flag GENPD_FLAG_ACTIVE_WAKEUP to consolidate this.
If specified, all slave devices configured as wakeup sources will be
kept active during system suspend.

PM Domains that need more fine-grained controls, based on the slave
device, can still provide their own callbacks, as before.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>