esoc: Protect the crash shutdown path against reboot notifier

SSR shutdown and reboot notifier both invoke clients hooks. These hooks
might be invoked at the same time leading to a race. Synchronize the
two paths.

Change-Id: Ibd0f7248956d60d5831068ffcf88895adb1cd30d
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>