dt-bindings: input: touchscreen: ilitek_ts_i2c: Absorb ili2xxx bindings

While Linux uses a different driver, the Ilitek
ILI210x/ILI2117/ILI2120/ILI251x touchscreen controller Device Tree
binding documentation is very similar.

  - Drop the fixed reg value, as some controllers use a different
    address,
  - Make reset-gpios optional, as it is not always wired.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/0c5f06c9d262c1720b40d068b6eefe58ca406601.1638539806.git.geert+renesas@glider.be