MAINTAINERS: add entry for the GISB arbiter driver

The GISB bus arbiter driver is relevant for BCM7xxx (brcmstb) platforms,
add an entry to cover this file.

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

MAINTAINERS

@@ -2093,6 +2093,7 @@ L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 F:	arch/arm/mach-bcm/*brcmstb*
 F:	arch/arm/boot/dts/bcm7*.dts*
+F:	drivers/bus/brcmstb_gisb.c
 
 BROADCOM TG3 GIGABIT ETHERNET DRIVER
 M:	Prashant Sreedharan <prashant@broadcom.com>