bus: brcmstb_gisb: Allow building as module

Allow building the Broadcom STB GISB arbiter driver as a module, however
similar to interrupt controller drivers, don't allow its unbind/removal
since it is not quite prepared for that and we want it to catch bus
errors all the time.

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