glink_ssr is used to signal a remote processor "A" about the stopping of
another remote processor "B", so that in the event that remote processor
B is ever booted again the fifos of the glink channel between A and B is
in a known state.

But if remote processor A receives this notification before B is
actually stopped the newly reset fifo indices will be interpreted as
there being "data" on the channel and either side of the channel will
enter a fatal error handler.

Move the glink_ssr notification to the "unprepare" state of the
rproc_subdev to avoid this issue.

This has the side effect of us not notifying the dying remote processor
itself about its fate, which has been seen to block in certain resource
constraint scenarios.
Tested-by: Sibi Sankar <sibis@codeaurora.org>
Reviewed-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>