PM / doc: Update device documentation for devices in IRQ-safe PM domains

Update documentation to reflect the changes made to support IRQ safe PM
domains.

Suggested-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>