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