bus: brcmstb_gisb: register the fault code hook

Commit 44127b77

 ("bus: add Broadcom GISB bus arbiter timeout/error
handler") added everything that is required to register an ARM fault
handler for imprecise external aborts, except that there is nothing
calling this currently.

We do not need to export that specific function and have to update
arch/arm/mach-bcm/brcmstb.c to call it, simply, register the fault
handler during the probe() function of the driver.

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