net: lwm2m: Refactoring of lwm2m_engine.c

Created the file lwm2m_registry.c to include the
getters and setters of values and the creation
and deletion of all lwm2m objects, resources
and instances. Refactored the registry part
of lwm2m_engine.h into lwm2m_registry.h.

Signed-off-by: Ola Tangen Kulseng <ola.kulseng@nordicsemi.no>