psm: detect Lenovo top-button clickpads, set INPUT_PROP_TOPBUTTONPAD evdev property

[val_packett.cool]

Summary

libinput has special handling for Lenovo ThinkPad *40 series, where it treats clicks on the top button area as if they came from the TrackPoint:
https://wayland.freedesktop.org/libinput/doc/latest/t440-support.html

Let's detect these devices and set the corresponding evdev property.

Test Plan

Tested on my X240.

[wulf]

Please assign hw.psm.synaptics.softbuttons_y sysctl some negative value so /dev/psm0 users get benefits from this change too.

[val_packett.cool]

Added softbuttons_y value for topbuttonpads.

Looking at r313757, I used to set softbuttons_y to -1700. Makes sense, the regular value is 1700.

BTW, looking back at my old notes from when I still used Xorg, xf86-input-synaptics with /dev/psm0 did its own synaptics-specific handling of button zones, configurable via Xorg.conf:

Option      "SoftButtonAreas" "4201 0 0 1950 2710 4200 0 1950"
Option      "AreaTopEdge" "5%"

[wulf]

I hope -1700 value is tested. It does not work for my "softbuttons bar at a bottom" laptop leaving softbutton area outside reportable region so I have to use -2200 to make it working. But "softbuttons bar at a top" touchpad should behave differently here.

[val_packett.cool]

In D18676#398820, @wulf wrote:

I hope -1700 value is tested.

Yes. I've had the value in my sysctl.conf for a couple years now. Just started moused to test in vt, the middle button at the top does paste :)