udev: keyboard-builtin: Add support for setting IBM trackpoint sensitivity

IBM / Lenovo trackpoints allow specifying a sensitivity setting through a
ps/2 command, which changes the range of the deltas sent when using the
trackpoint.

On some models with normal usage only deltas of 1 or 2 are send, resulting in
there only being 2 mouse cursor movement speeds, rather than the expected fluid
scale. Changing the sensitivity to a higher level than the bootup default fixes
this.

This commit adds support for setting a POINTINGSTICK_SENSITIVITY value
in hwdb to allow changing the sensitivity on boot through udev / hwdb.

hwdb/70-pointingstick.hwdb

@@ -41,6 +41,7 @@
 #
 # Allowed properties are:
 #   POINTINGSTICK_CONST_ACCEL
+#   POINTINGSTICK_SENSITIVITY
 #
 #########################################
 #      POINTINGSTICK_CONST_ACCEL        #
@@ -53,6 +54,17 @@
 # by how much to multiply deltas generated by the pointingstick to get
 # normalized deltas.
 #
+#########################################
+#      POINTINGSTICK_SENSITIVITY        #
+#########################################
+#
+# TPPS/2 IBM TrackPoint driver sensitivity sysfs setting
+#    POINTINGSTICK_SENSITIVITY=<sensitivity>
+#
+# Where <sensitivity> is a number between 0 and 255, note this property
+# only applies to TPPS/2 IBM TrackPoint devices, see
+# drivers/input/mouse/trackpoint.c in the Linux kernel sources.
+#
 
 #
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