ctrl/abis: When the max_power_reduction changes, send it to the BTS

In case the max_power_reduction changes, issue a new Set Radio
Carrier Attributes command. OML 12.21 allows to not include the
ARFCN list and the semantic I picked/understand is that a partial
update is possible.

Fixes: SYS#267
diff --git a/openbsc/include/openbsc/abis_nm.h b/openbsc/include/openbsc/abis_nm.h
index 4d6f866..965f158 100644
--- a/openbsc/include/openbsc/abis_nm.h
+++ b/openbsc/include/openbsc/abis_nm.h
@@ -183,4 +183,7 @@
 			struct abis_nm_sw_descr *res, const int res_len);
 int abis_nm_select_newest_sw(const struct abis_nm_sw_descr *sw, const size_t len);
 
+/* Helper functions for updating attributes */
+int abis_nm_update_max_power_red(struct gsm_bts_trx *trx);
+
 #endif /* _NM_H */