soc: qcom: ssr: Export find_subsys_device function

Export the find_subsys() SSR API for the clients to get the
subsystem device descriptor for a particular subsystem.

Change-Id: I8aa2e7b8c44c3682c1dbf7e5fca75680c469690f
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>