MAINTAINERS: add a git entry for BCM7xxx ARM-based SoCs

Use github.com/broadcom/stblinux.git as our default development tree for
Broadcom BCM7xxx ARM-based SoCs.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>

MAINTAINERS

@@ -2141,6 +2141,7 @@ M:	Brian Norris <computersforpeace@gmail.com>
 M:	Gregory Fong <gregory.0xf0@gmail.com>
 M:	Florian Fainelli <f.fainelli@gmail.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
+T:	git git://git.github.com/broadcom/stblinux.git
 S:	Maintained
 F:	arch/arm/mach-bcm/*brcmstb*
 F:	arch/arm/boot/dts/bcm7*.dts*