dt-bindings: iio: Add ltc2983 documentation

Document the LTC2983 temperature sensor devicetree bindings.

Tweaked by Jonathan to take into account the lack of signed output
being maintained by dtc yaml output.  For now a comment added
that the unsigned array should actually be signed.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>