powerpc: Update MAINTAINERS to point at shared tree

Now that we have a shared powerpc tree on kernel.org, point folks at that
as the primary place to look for powerpc stuff.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

MAINTAINERS

@@ -6161,7 +6161,7 @@ M:	Michael Ellerman <mpe@ellerman.id.au>
 W:	http://www.penguinppc.org/
 L:	linuxppc-dev@lists.ozlabs.org
 Q:	http://patchwork.ozlabs.org/project/linuxppc-dev/list/
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/benh/powerpc.git
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git
 S:	Supported
 F:	Documentation/powerpc/
 F:	arch/powerpc/