iio: afe: rescale: add temperature transducers

A temperature transducer is a device that converts a thermal quantity
into any other physical quantity. This patch adds support for
temperature to voltage (like the LTC2997) and temperature to current
(like the AD590) linear transducers.
In both cases these are assumed to be connected to a voltage ADC.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-9-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>