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: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

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

···
       - 2 # ATMEL_MXT_WAKEUP_GPIO
     default: 0
 
+  wakeup-source:
+    type: boolean
+
 required:
   - compatible
   - reg