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 <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>