dt-bindings: hwmon: tmp108: add optional interrupts and #thermal-sensor-cells

The tmp108 does have an alert output that can be used as interrupt source
and can of course also be used as part of a thermal sensor setup for things
like thermal-based cpu frequencies, so document the necessary properties.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>