remoteproc: stm32_rproc: Add mutex protection for workqueue

The workqueue may execute late even after remoteproc is stopped or
stopping, some resources (rpmsg device and endpoint) have been
released in rproc_stop_subdevices(), then rproc_vq_interrupt()
accessing these resources will cause kernel dump.

Call trace:
virtqueue_add_inbuf
virtqueue_add_inbuf
rpmsg_recv_single
rpmsg_recv_done
vring_interrupt
stm32_rproc_mb_vq_work
process_one_work
worker_thread
kthread

Suggested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20230331160634.3113031-1-arnaud.pouliquen@foss.st.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>