iio: adc: ad4000: Add timestamp channel

The ADC data is pushed to the IIO buffer along with timestamp but no
timestamp channel was provided to retried the time data.
Add a timestamp channel to provide sample capture time.

Suggested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/d1f1bb1b726b90a3a7c1148c65d2f7fe073e2b15.1733147444.git.marcelo.schmitt@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>