dt-bindings: remoteproc: stm32-rproc: add new mailbox channel for detach

Add the "detach" mailbox item, that allows to define a mailbox to
send a IPCC signal to the remote processor on remoteproc detach action.
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210331073347.8293-2-arnaud.pouliquen@foss.st.comSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/devicetree/bindings/remoteproc/st,stm32-rproc.yaml

@@ -65,16 +65,23 @@ properties:
           Unidirectional channel:
             - from local to remote, where ACK from the remote means that it is
               ready for shutdown
+      - description: |
+          A channel (d) used by the local proc to notify the remote proc that it
+          has to stop interprocessor communnication.
+          Unidirectional channel:
+            - from local to remote, where ACK from the remote means that communnication
+              as been stopped on the remote side.
     minItems: 1
-    maxItems: 3
+    maxItems: 4
 
   mbox-names:
     items:
       - const: vq0
       - const: vq1
       - const: shutdown
+      - const: detach
     minItems: 1
-    maxItems: 3
+    maxItems: 4
 
   memory-region:
     description: