dt-bindings: iio: temperature: ltc2983: change default excitation for custom thermistors

Excitation cannot be set to auto-range when using a custom thermistor
or Steinhart sensor type. Default it to 1000nA to match the driver
and remove the auto-range value from the enum.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221103130041.2153295-9-demonsingur@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>