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.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>