dt-bindings: input: touchscreen: edt-ft5x06: Add 'threshold' property

Add a new property 'threshold' to the edt-ft5x06 touchscreen binding.
This property allows setting the "click"-threshold in the range from 0
to 255. This change addresses the following dtbs_check warning:
imx6dl-lanmcu.dtb: touchscreen@38: 'threshold' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230621093245.78130-6-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>