net: lwm2m: Add helper macro for filling lwm2m_obj_path structs

Allows filling up struct lwm2m_obj_path by a macro.
For example:
  struct lwm2m_obj_path p1 = LWM2M_OBJ(MY_OBJ);
  struct lwm2m_obj_path p2 = LWM2M_OBJ(MY_OBJ, 0, RESOURCE);

Similarly, some function calls accept the structure, so it can
be initialized from stack and given by a pointer
  lwm2m_notify_observer_path(&LWM2M_OBJ(obj_id, 0, RESOURCE_ID));

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>