Move CU-UP related processing to separate file

A later commit will introduce an RB tree to manage multiple CU-UPs. The
RB tree implementation relies on some macros that generate RB tree
functions. Functions using the RB tree implementation will be grouped in
this file.

CMakeLists.txt

@@ -1277,6 +1277,7 @@ set(L2_NR_SRC
   ${NR_RRC_DIR}/rrc_gNB_UE_context.c
   ${NR_RRC_DIR}/rrc_gNB_NGAP.c
   ${NR_RRC_DIR}/rrc_gNB_radio_bearers.c
+  ${NR_RRC_DIR}/rrc_gNB_cuup.c
   )
 
 set(L2_SRC_UE