dt-bindings: iio: temperature: ltc2983: require custom sensor tables

The driver will error out when a custom sensor type is used but a
custom sensor table is not provided. Require it in the binding too.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221103130041.2153295-7-demonsingur@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>