staging: iio: ad2s1210: Add device tree table.

Add device tree table for matching vendor ID.

Note that as the driver is still heavily in flux and there are some
non obvious aspects to a comprehensive binding, that binding will
be submitted at a later date (before leaving staging!)

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>