soc: qcom: ssr: Wait for smp2p ack after sending sysmon shutdown message

After sending a sysmon shutdown request to the SSCTL service on the
subsystem, wait for the service to set a shutdown_ack smp2p bit in
addition to sending the indication message back.

Change-Id: I231607a8ade983c9a2c2c0a39d53023ced6e81fb
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>