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

Add a git tree entry for the BMIPS-based BCM7xxx SoCs located at
github.com/broadcom/stblinux.git.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>

MAINTAINERS

@@ -2151,6 +2151,7 @@ BROADCOM BMIPS MIPS ARCHITECTURE
 M:	Kevin Cernekee <cernekee@gmail.com>
 M:	Florian Fainelli <f.fainelli@gmail.com>
 L:	linux-mips@linux-mips.org
+T:	git git://git.github.com/broadcom/stblinux.git
 S:	Maintained
 F:	arch/mips/bmips/*
 F:	arch/mips/include/asm/mach-bmips/*