net: lwm2m: refactor engine_get_obj to be more useful

Eliminate several similar code-blocks by replacing
engine_get_resource() with a more useful function called:
path_to_objs()

By supplying an lwm2m_obj_path object, it will find and set
the related values for:
struct lwm2m_engine_obj_inst
struct lwm2m_engine_obj_field
struct lwm2m_engine_res_inst

NOTE: NULLs can be supplied where the returned value is not
important.

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