introduce dedicated nr pdcp tick thread for F1 split

Currently, NR PDCP ticks are active only with monolithic gNB, triggered by the MAC scheduler in gNB_scheduler.c.
In F1 split mode, as there is no MAC, there’s no source to advance the NR PDCP timer via "nr_pdcp_wakeup_timer_thread()".
This commit introduces a dedicated thread for NR PDCP ticks in gNB-CU and CU-UP mode.
Without it, packet losses at the F1 uplink would cause indefinite buffering, as t-Reordering won't expire due to the lack of NR PDCP timer ticks.

executables/nr-softmodem.c

@@ -532,7 +532,7 @@ void init_pdcp(void) {
   uint32_t pdcp_initmask = IS_SOFTMODEM_NOS1 ? ENB_NAS_USE_TUN_BIT : LINK_ENB_PDCP_TO_GTPV1U_BIT;
 
   if (!NODE_IS_DU(get_node_type())) {
-    nr_pdcp_layer_init(get_node_type() == ngran_gNB_CUCP);
+    nr_pdcp_layer_init();
     nr_pdcp_module_init(pdcp_initmask, 0);
   }
 }