bus: brcmstb_gisb: add notifier handling

Check for GISB arbitration errors through a chained notifier
when a process dies or a kernel panic occurs.  This allows a
meaningful GISB diagnostic message to occur along with other
diagnostic information from the event.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>