HID: sony: Make the DS4 touchpad a separate device

The dualshock 4 supports both analog sticks of which one uses
ABS_X/_Y and a touchpad. In a recent discussion with Dmitry about
some input-mt changes we proposed for disabling pointer emulation from
input_mt_sync_frame, Dmitry mentioned ABS_X/_Y should report the
same data as ABS_MT_POSITION_X/_Y. The current driver is mixing axes
for different subdevices. It was suggested to make the touchpad
its own sub-device.

This patch turns the touchpad into its own device. In addition
this patch also moves the button underneath the touchpad into
the new device. It felt like this button should be part of the
device. No known user space application (not even SDL2) seems to
be using it.

Signed-off-by: Roderick Colenbrander <roderick.colenbrander@sony.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>