net: lwm2m: mark OPTIONAL resources for LwM2M Device

Using OMA Technical Specification LwM2M Enabler 1.0.2 dated
Feb. 9, 2018, let's mark the OPTIONAL resources for an LwM2M
Device object (Section E.4 "LwM2M Object: Device")

As a result, the Device object no longer configures the default
buffers for data storage of several optional resources.
The LwM2M client sample is also changed to to setup these read-only
buffers instead.

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