PM / QoS: Add notifier for flags

dev_pm_qos has a notifier for DEV_PM_QOS_LATENCY. Add a similar
notifier for DEV_PM_QOS_FLAGS.

Bug 1364240

Change-Id: Ica4c58708855938818a1e75896503b9023b96573
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/288810