ARM: dts: axp209: Add iio-hwmon node for internal temperature

This adds a DT node to hook up the internal temperature ADC to the
iio-hwmon driver. The various voltage + current ADCs are consumed and
exposed by their respective drivers, but this is not and is always
available. Naming chosen to match the axp20x_ prefix the power sensors
use.

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/2a9bd53a65c4a154cccba622c60cbffcdceaeb95.1683719613.git.noodles@earth.li


Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>