soc: qcom: subsys_notif: Add support for early SSR notifications

Currently, the SSR framework sends subsystem state transtition
notifications during SSR to inform interested clients about
the subsystem's state. However, there may be clients of the
notification framework that would like to know that a
subsystem has gone down, and before that subsystem has started
undergoing SSR, so that they can ensure that communication
towards a downed subsystem ceases.

Add support for registering callbacks to be invoked as soon as
a subsystem crashes, and before commencing SSR.

Change-Id: Ia3ce647a7b1167b44ab9c97b4b0b0637e204b116
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>