MAINTAINERS: Add keystone git tree information

Update the Keystone entry to add git tree information.
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>

MAINTAINERS

@@ -1013,6 +1013,7 @@ M:	Santosh Shilimkar <santosh.shilimkar@ti.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 F:	arch/arm/mach-keystone/
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/ssantosh/linux-keystone.git
 
 ARM/LOGICPD PXA270 MACHINE SUPPORT
 M:	Lennert Buytenhek <kernel@wantstofly.org>