bssgp_bvc_fsm: Add a hook to notify when a reset was acknowledged

There are cases where we want to be notified of a successful BVC reset,
e.g. for a signalling because we can then start resetting the PtP-BVCs.
With this hook it's now possible to do that.

Change-Id: If240dd13f0f674693018c93390386b2c8afb97af
Related: SYS#5908
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 8902371..a6d16d2 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what			description / commit summary line
 libosmogsm      ABI BREAKAGE            CELL_IDENT_WHOLE_GLOBAL_PS changed enum number
 libosmogsm	add struct member	Add codec_list_bss_supported to gsm0808_handover_request_ack (more_items flag ensures ABI compat)
+libosmogb	ABI BREAKAGE		Add reset_ack_notification function pointer to struct bssgp_bvc_fsm_ops