hwdb: add a touchpad hwdb

Currently used to tag devices in the new Lenovo *50 series and the X1 Carbon
3rd. These laptops re-introduced the physical trackpoint buttons that were
missing from the *40 series but those buttons are now wired up to the
touchpad.

The touchpad now sends BTN_0, BTN_1 and BTN_2 for the trackpoint. The same
button codes were used in older touchpads that had dedicated scroll up/down
buttons. Input drivers need to work around this and thus know what they're
dealing with.

For the previous gen we introduced INPUT_PROP_TOPBUTTONPAD in the kernel, but
the resulting mess showed that these per-device quirks should really live in
userspace.

The list currently includes the X1 Carbon 3rd PNPID, others will be added as
get to know which PNPID they have.