dt-bindings: HID: i2c-hid: elan: Introduce Elan eKTH8D18

The Elan eKTH8D18 touchscreen controller is an I2C HID device with a
longer boot-up time. Power sequence timing wise it is compatible with
the eKTH6A12NAY, with a power-on delay of at least 5ms, 20ms
out-of-reset for I2C ack response, and 150ms out-of-reset for I2C HID
enumeration, both shorter than what the eKTH6A12NAY requires.
Enumeration and subsequent operation follows the I2C HID standard.

Add a compatible string for it with the ekth6a12nay one as a fallback.
No enum was used as it is rare to actually add new entries. These
chips are commonly completely backward compatible, and unless the
power sequencing delays change, there is no real effort being made to
keep track of new parts, which come out constantly.

Also drop the constraints on the I2C address since it's not really
part of the binding.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20250617082004.1653492-2-wenst@chromium.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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