netlink: specs: add phys-binding attr to rt_link spec

Add the missing phys-binding attr to the mctp-attrs in the rt_link spec.
This fixes commit 580db51 ("net: mctp: Expose transport binding
identifier via IFLA attribute").

Note that enum mctp_phys_binding is not currently uapi, but perhaps it
should be?

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20241213112551.33557-1-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt_link.yaml

@@ -2086,6 +2086,9 @@ attribute-sets:
       -
         name: mctp-net
         type: u32
+      -
+        name: phys-binding
+        type: u8
   -
     name: stats-attrs
     name-prefix: ifla-stats-