Documentation: Add AD4000 documentation

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/aeee5fd9deccf85beadecf58c9b938b97a3aeba5.1720810545.git.marcelo.schmitt@analog.com


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