iio: imu: adis16480: Treat temperature scale in a generic way

All supported devices provide internal temperature measurement from -40 C
to +85 C, with +25 C representing value 0x00.

This patch treats the temperature scale in a generic way, similar to the
accelerometer and gyroscope scales. So far, there are no temperature max
scale differences between the supported devices. However, devices that
will make use of this feature will be added in the future.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>