net: lwm2m: add IPSO support w/ temperature sensor object

IPSO Smart Objects are a set of template objects based on the LwM2M
object framework which are designed to represent standard hardware
such as temperature and humidity sensors or light controls.

Let's add a place for these objects to live as well as an initial
temperature sensor object.

Signed-off-by: Michael Scott <michael.scott@linaro.org>