dt-bindings: ads7846: Add hsync-gpios

The TI ADS7846 emits a horizontal sync signal that is usually
connected to a GPIO for polling. Add a binding for this.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240430-gpio-leds-miscarm-v1-2-9c94d7711f6c@linaro.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/ads7846.txt

@@ -57,6 +57,7 @@ Optional properties:
 					pendown-gpio (u32).
 	pendown-gpio			GPIO handle describing the pin the !PENIRQ
 					line is connected to.
+	ti,hsync-gpios			GPIO line to poll for hsync
 	wakeup-source			use any event on touchscreen as wakeup event.
 					(Legacy property support: "linux,wakeup")
 	touchscreen-size-x		General touchscreen binding, see [1].