PM / input / touchscreen: Make st1232 use device PM QoS constraints

Make the st1232 driver use dev_pm_qos_add_ancestor_request() to
add a device PM QoS latency constraint for the controller it
depends on, so that the controller won't go into an overly deep
low-power state when the touchscreen has to be particularly
responsive (e.g. when the user moves his or her finger on it).

This change is based on a prototype patch from Guennadi Liakhovetski.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>