iio: adc: ad7124: add ext attributes to temperature channel

iio: adc: ad7124: add ext attributes to temperature channel

Use the same .ext_info for the temperature channel as for the voltage
channels. In the ADC, these all go though a mux to select the source
and otherwise operate the same.

These attributes probably won't be used much, but since it is trivial
to add this, we might as well include them.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>