remoteproc: stm32: add capability to detach

A mechanism similar to the shutdown mailbox signal is implemented to
detach a remote processor.

Upon detachment, a signal is sent to the remote firmware, allowing it
to perform specific actions such as stopping rpmsg communication.

The Cortex-M hold boot is also disabled to allow the remote processor
to restart in case of crash.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210331073347.8293-3-arnaud.pouliquen@foss.st.com


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