dt-bindings: input: touchscreen: ilitek_ts_i2c: Add bindings

Add binding documentation for ILITEK touch devices.

Signed-off-by: Joe Hung <joe_hung@ilitek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210324122601.125873-1-joe_hung@ilitek.com


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