Paging (MAC): schedule PCCH in DL path at PF/PO

At each DL slot, schedule_nr_pcch() scans pcch_queue for records whose
UE paging occasion matches (frame, slot), encodes one PCCH-Message via
do_NR_Paging(), and fills P-RNTI PDCCH/PDSCH NFAPI PDUs.

Paging occasion detection is now owned by MAC via PCCH scheduler
helper that implements PF/PO computation per TS 38.304
§7. Computation is determined by defaultPagingCycle (T),
nAndPagingFrameOffset (N), ns (Ns), with PF formula
(SFN + PF_offset) mod T = (T div N) * (UE_ID mod N),
and PO slot mapping for Ns = 1/2/4. Paging is transmitted only
when (frame, slot) is the UE's paging occasion
(TS 38.304 §7.1) and when a Type2-PDCCH CSS is configured via
pagingSearchSpace, so the UE can receive it.

The RRC PCCH path is simplified accordingly.

Changes

In `openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_pcch.c`:
- Add paging-specific scheduling logic, including `is_paging_occasion()`,
  paging-search-space resolution, and `nr_fill_nfapi_dl_PCCH_pdu()`
- Add `schedule_nr_pcch()` implementation to allocate CCE/PDSCH
  resources, populate DL/TX NFAPI requests, encode at PO via do_NR_Paging()

In gNB scheduler:
- Invoke `schedule_nr_pcch()` in SA-mode DL scheduling

In `openair2/LAYER2/NR_MAC_COMMON/nr_mac_common.c`:
- Add PCCH scheduling helpers to be reused at UE side

Also:
- Remove no longer relevant `rrc_gNB_generate_pcch_msg()` from `rrc_gNB.c`

Signed-off-by: Guido Casati <guido.casati@openairinterface.org>

common/platform_constants.h

@@ -49,6 +49,9 @@
                                        * defined for NGAP in TS 38.413 - maxnoofDRBs */
 #define MAX_PDUS_PER_UE         (8)   /* Maximum number of PDU Sessions per UE */
 
+/** Maximum value of nrofPDCCH-MonitoringOccasionPerSSB-InPO-r16 (TS 38.331 PCCH-Config) */
+#define NR_PCCH_MAX_MO_PER_SSB_IN_PO 4
+
 #define NB_RB_MBMS_MAX (29 * 16) /* 29 = LTE_maxSessionPerPMCH + 16 = LTE_maxServiceCount from LTE_asn_constant.h */
 
 #define NB_RAB_MAX 11 /* from LTE_maxDRB in LTE_asn_constant.h */