iio: temperature: Add support for Azoteq IQS620AT temperature sensor

This patch adds support for the Azoteq IQS620AT temperature sensor,
capable of reporting its absolute die temperature.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>