MCTP.Endpoint: Document configured_by association

The `configured_by` / `configures` association for MCTP endpoints is
hosted by mctpreactor[1]. It provides a mechanism to traverse backwards
to Entity Manager configuration objects from the D-Bus objects hosted
by mctpd[2].

[1]: https://gerrit.openbmc.org/c/openbmc/dbus-sensors/+/69111
[2]: https://github.com/codeconstruct/mctp

Change-Id: I33b9887cf26519a3d3ac4d6c084e5852ba134163
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/yaml/xyz/openbmc_project/MCTP/Endpoint.interface.yaml b/yaml/xyz/openbmc_project/MCTP/Endpoint.interface.yaml
index 568fb99..d867fa2 100644
--- a/yaml/xyz/openbmc_project/MCTP/Endpoint.interface.yaml
+++ b/yaml/xyz/openbmc_project/MCTP/Endpoint.interface.yaml
@@ -39,3 +39,13 @@
           DSP0239 specification. A device that supports a given message type may
           not support that message type equally across all buses that connect to
           the device.
+
+associations:
+    - name: configured_by
+      description: >
+          Endpoints exposed by platform configuration should host a
+          configured_by assocation back to the object that drove the endpoint's
+          existence.
+      reverse_name: configures
+      required_endpoint_interfaces:
+          - xyz.openbmc_project.Configuration.MCTPDevice