bus: brcmstb_gisb: Make the driver buildable on MIPS

BCM7xxx ARM and MIPS platforms share a similar hardware block for
reporting GISB errors, so they both benefit from the use of this driver.
Conditionally compile the ARM-specific bus error handler so that the
GISB error IRQ handler works on other architectures.

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