dt-bindings: iio: Add AD7091R-8

Add device tree documentation for AD7091R-8.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/632eb34801ae54feda453b6a65d60fc8ac2891fd.1703013352.git.marcelo.schmitt1@gmail.com


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