bus: brcmstb_gisb: enable driver for ARM64 architecture

The ARM64 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>