soc: qcom: glink_ssr: Update SSR to handle absent subsystems

Currently, the SSR module attempts to notify subsystems which
are not part of restart events. This leads to needless restarts
when the absent subsystems do not respond.

Update the SSR module to properly handle absent subsystems.

Change-Id: Ie4ca7e75cb3cb103a6fff94984c6993ef7f4a756
Signed-off-by: Steven Cahail <scahail@codeaurora.org>
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>