PM / Domains: Don't warn about IRQ safe device for an always on PM domain

When an IRQ safe device is attached to a no sleep domain, genpd prints a
warning once, as to indicate it is a suboptimal configuration from power
consumption point of view.

However the warning doesn't make sense for an always on domain, since it
anyway remains powered on. Therefore, let's change to not print the warning
for this configuration.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>