bus: brcmstb_gisb: remove low-level ARM hooks

The ARM architecture can provide meaningful diagnostic
output from the GISB arbiter solely from interrupts and
notifiers without the need to hook the low level fault
handlers.

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