bus: add Broadcom GISB bus arbiter timeout/error handler

This patch adds support for the Broadcom GISB arbiter bus timeout/error
handler. GISB is a proprietary bus used by Broadcom Set Top Box
System-on-a-chip devices (BCM7xxx) which allows multiple masters and
clients to be interfaced with each other.

The bus arbiter offers support for generating two interrupts towards the
host CPU, thus allowing us to "catch" clock gated masters, or masters
being volontarily blocked for powersaving purposes, or do general system
troubleshooting.

We also register a hook with the ARM fault exception handling to allow
printing a more informative message than "imprecise external abort at
0x00000000" for instance.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>