net: lwm2m: mark OPTIONAL resources for IPSO Light Control

Using IPSO Smart Object Guideline: "Smart Objects Starter Pack 1.0"
dated May 27, 2017, let's mark the OPTIONAL resources for an
IPSO Light Control object (Section 16. "IPSO Object: Light Control")

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>