Add stats: pcu.bts.N.pdch.available/occupied

Count available PDCHs (3GPP TS 52.402 § B.2.1.38) as well as occupied
PDCHs (§ B.2.1.42-44).

Related: SYS#4878
Change-Id: I74760a68ee055510a79e80854ec7bf1521669119
diff --git a/src/bts.h b/src/bts.h
index 5e45527..a6e7150 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -182,6 +182,8 @@
 
 enum {
 	STAT_MS_PRESENT,
+	STAT_PDCH_AVAILABLE,
+	STAT_PDCH_OCCUPIED,
 };
 
 /* RACH.ind parameters (to be parsed) */
@@ -346,6 +348,10 @@
 	osmo_stat_item_set(item, val + inc);
 }
 
+#define bts_stat_item_inc(bts, stat_id) bts_stat_item_add(bts, stat_id, 1)
+
+#define bts_stat_item_dec(bts, stat_id) bts_stat_item_add(bts, stat_id, -1)
+
 struct gprs_rlcmac_bts *bts_alloc(struct gprs_pcu *pcu, uint8_t bts_nr);
 
 struct gprs_rlcmac_sba *bts_alloc_sba(struct gprs_rlcmac_bts *bts, uint8_t ta);