dt-bindings: input: atmel,maxtouch: add wakeup-source

The touchscreen can be a wake up source and it's being used in DTS:

  arch/arm/boot/dts/exynos5250-spring.dt.yaml:
    trackpad@4b: 'wakeup-source' does not match any of the regexes: 'pinctrl-[0-9]+'
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210212163806.69996-1-krzk@kernel.orgSigned-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/atmel,maxtouch.yaml

@@ -80,6 +80,9 @@ properties:
       - 2 # ATMEL_MXT_WAKEUP_GPIO
     default: 0
 
+  wakeup-source:
+    type: boolean
+
 required:
   - compatible
   - reg