MIPS kernels allow platforms to invoke a custom Bus Error handler, add the
necessary code to do this for Broadcom SoCs where the GISB bus error handler can be used.

We may get a bus error from an address decoded outside of the GISB bus space,
so we need to check the validity of such a capture before printing anything.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: cernekee@gmail.com
Cc: arnd@arndb.de
Cc: jaedon.shin@gmail.com
Cc: pgynther@google.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12284/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>