net: lwm2m: support senml cbor object link

use "vlo" map key

Signed-off-by: Pascal Brogle <pascal.brogle@husqvarnagroup.com>

subsys/net/lib/lwm2m/lwm2m_senml_cbor.cddl

@@ -9,7 +9,8 @@ record = {
 	    vf => float       // ; Float Value
 	    vs => tstr        // ; String Value
 	    vb => bool        // ; Boolean Value
-	    vd => bstr ),        ; Data Value
+	    vd => bstr        // ; Data Value
+	    vlo => tstr ),       ; Object Link Value
 	0*5 key-value-pair       ; To handle unordered maps; length-first ordered map keys
 }
 
@@ -27,3 +28,4 @@ vf = 2
 vs = 3
 vd = 8
 vb = 4
+vlo = "vlo"