dt-bindings: adc-joystick: add poll-interval

Add poll-interval support for the adc-joystick documentation. This is
an optional value and if not provided the adc-joystick works as it
does today (with buffers). If this value is provided, the adc-joystick
driver is polled at the specified interval. The existing attribute of
"poll-interval" was used instead of complying with property-units.yaml
after discussion of the issue on the mailing list.

Signed-off-by: Maya Matuszczyk <[e-mail]>
Signed-off-by: Chris Morgan <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Acked-by: Artur Rojek <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/adc-joystick.yaml

···
   Bindings for joystick devices connected to ADC controllers supporting
   the Industrial I/O subsystem.
 
+allOf:
+  - $ref: input.yaml#
+
 properties:
   compatible:
     const: adc-joystick
···
       See
       https://github.com/devicetree-org/dt-schema/blob/master/schemas/iio/iio-consumer.yaml
       for details.
+
+  poll-interval: true
 
   '#address-cells':
     const: 1