fix: amf determine ue.rattype in registration

[donald1218]

In TS 23.502 section 4.2.2.1, it is mentioned

the AMF determines the Access Type and RAT Type as defined in clause 5.3.2.3 of TS 23.501 [2]. 

However, the current AMF has not yet determined ue.rattype, which has caused ue.rattype to remain empty.

---

In TS 23.501 section 5.3.2.3 Registration Area management

For 3GPP access the AMF determines the RAT type the UE is camping on based on the Global RAN Node IDs 
associated with the N2 interface and additionally the Tracking Area indicated by NG-RAN.

---

Global RAN Node ID is define in TS 38.413