PM QoS: Generalize and export constraints management code
In preparation for the per-device constratins support:
- rename update_target to pm_qos_update_target
- generalize and export pm_qos_update_target for usage by the upcoming
per-device latency constraints framework:
* operate on struct pm_qos_constraints for constraints management,
* introduce an 'action' parameter for constraints add/update/remove,
* the return value indicates if the aggregated constraint value has
changed,
- update the internal code to operate on struct pm_qos_constraints
- add a NULL pointer check in the API functions
Signed-off-by: Jean Pihet <j-pihet@ti.com> Reviewed-by: Kevin Hilman <khilman@ti.com> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>