dt-bindings: iio: temperature: ltc2983: Make 'adi,custom-thermocouple' signed

The 'adi,custom-thermocouple' property is signed based on the example
and driver, so it's type should be int64-matrix rather than
uint64-matrix.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230404205014.644336-2-robh@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>