net: lwm2m: add missing application type to IPSO Light Control

Per IPSO Light Control definition from the OMA LwM2M registry:
http://www.openmobilealliance.org/tech/profiles/lwm2m/3311.xml



There is an optional "Application Type" string resource (5750) in the
Light Control object.  This was missed in the initial implementation.

NOTE: sample will assign reference if needed.

Signed-off-by: Michael Scott <mike@foundries.io>