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

Documentation/devicetree/bindings/iio/temperature/adi,ltc2983.yaml

@@ -127,7 +127,7 @@ patternProperties:
         description:
           Used for digitizing custom thermocouples.
           See Page 59 of the datasheet.
-        $ref: /schemas/types.yaml#/definitions/uint64-matrix
+        $ref: /schemas/types.yaml#/definitions/int64-matrix
         minItems: 3
         maxItems: 64
         items: