remoteproc: qcom: Wait for shutdown-ack/ind on sysmon shutdown

After sending a sysmon shutdown request to the SSCTL service on the
subsystem, wait for the service to send shutdown-ack interrupt or
an indication message to signal the completion of graceful shutdown.

Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>