API: Add binding accessors to generic struct mctp_binding

Rather than have every binding include their own wrappers around
binding<->core functions, introduce an accessor to retrieve the struct
mctp_binding from each.

This means we no longer need the binding-specific registration
callbacks. However, we do now need a ->start callback, to allow bindings
to perform post-registration init.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Change-Id: I6cee9e93f37520f85c155a0ca34017cc0675552c
diff --git a/libmctp-serial.h b/libmctp-serial.h
index 5504864..5eaf948 100644
--- a/libmctp-serial.h
+++ b/libmctp-serial.h
@@ -13,8 +13,7 @@
 
 struct mctp_binding_serial *mctp_serial_init(void);
 
-void mctp_serial_register_bus(struct mctp_binding_serial *serial,
-		struct mctp *mctp, mctp_eid_t eid);
+struct mctp_binding *mctp_binding_serial_core(struct mctp_binding_serial *b);
 
 /* file-based IO */
 int mctp_serial_get_fd(struct mctp_binding_serial *serial);