dt-bindings: touchscreen: move ar1021.txt to trivial-touch.yaml

ar1021 have only reg and interrupts property beside touch common
properties. So move context of ar1021.txt into trivial-touch.yaml.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250926184720.391335-2-Frank.Li@nxp.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: nxpfrankli

Documentation/devicetree/bindings/input/touchscreen/ar1021.txt

@@ -14,6 +14,8 @@ properties:
     enum:
       # MAXI MAX11801 Resistive touch screen controller with i2c interface
       - maxim,max11801
+      # Microchip AR1020 and AR1021 touchscreen interface (I2C)
+      - microchip,ar1021-i2c
 
   reg:
     maxItems: 1