staging:iio:ad2s90: Add device tree support

This patch adds device tree support to ad2s90 with standard
device tree id table.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>