bus: brcmstb_gisb: resolve section mismatch

Commit f1bee783 moved the call to hook_fault_code in
brcmstb_gisb_arb_probe() which now calls a function annotated with __init, so
this one must also be annotated with __init.

In order to avoid introducing another section mismatch, call
platform_driver_probe() manually and remove the .probe assignment from
brcmstb_gisb_arb_driver, this is very similar to what
drivers/pci/host/pci-imx6.c does since we basically have the same constraints
here.

Fixes: f1bee783

 ("bus: brcmstb_gisb: register the fault code hook")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>