dt-bindings: input: touchscreen: elants_i2c: Add compatible for eth3915n chip

This adds a new compatible string for Elan eth3915n touchscreen controller,
which is compatible with ekth3500.

Signed-off-by: Yunlong Jia <ecs.beijing2022@gmail.com>
Suggested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20220923083657.v5.2.Ic4e8f03868f88b8027a81bc3d414bae68978e6b7@changeid


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