Also adds a class type PM_QOS_SUM that aggregates the values by summing them.

It can be used by memory controllers to calculate the optimum clock frequency
based on the bandwidth needs of the different memory clients.
Signed-off-by: NTomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>