input: mt: add the ABS_MT_PRESSURE event

For pressure-based multi-touch devices, a direct way to send sensor
intensity data per finger is needed. This patch adds the ABS_MT_PRESSURE
event to the MT protocol.

Requested-by: Yoonyoung Shim <jy0922.shim@xxxxxxxxxxx>
Requested-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
Requested-by: Peter Hutterer <peter.hutterer@xxxxxxxxx>
Signed-off-by: Henrik Rydberg <rydberg@xxxxxxxxxxx>