dt-bindings: iio: vadc: Add unit address to ADC channel node in example

The node has a reg property, therefore its name should include a unit
address. Also change the name from 'usb_id_nopull' to 'adc-chan', which
is the preferred name for ADC channel nodes.

Include headers for constants used in the example.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>