dt-bindings: add more optional properties for elan_i2c touchpads

Some new touchpads IC are connected through PS/2 and I2C. On some of these
new IC, the I2C part doesn't have all of the information available.
We need to be able to forward the touchpad parameters from PS/2 and
thus, we need those new optional properties.

Signed-off-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/elan_i2c.txt

···
   pinctrl binding [1]).
 - vcc-supply: a phandle for the regulator supplying 3.3V power.
 - elan,trackpoint: touchpad can support a trackpoint (boolean)
+- elan,clickpad: touchpad is a clickpad (the entire surface is a button)
+- elan,middle-button: touchpad has a physical middle button
+- elan,x_traces: number of antennas on the x axis
+- elan,y_traces: number of antennas on the y axis
+- some generic touchscreen properties [2]:
+  * touchscreen-size-x
+  * touchscreen-size-y
+  * touchscreen-x-mm
+  * touchscreen-y-mm
+
 
 [0]: Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
 [1]: Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt
+[2]: Documentation/devicetree/bindings/input/touchscreen/touchscreen.txt
 
 Example:
 	&i2c1 {