MAINTAINERS: Add Wei Liu as toolstack co-maintainer.

The three existing maintainers are not really able to keep up with
the flow and Wei is one of the top tools contributors (according to
"git shortlog -s -n -p RELEASE-4.4.0..origin/staging tools" and my
own impressions).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

MAINTAINERS

@@ -283,6 +283,7 @@ TOOLSTACK
 M:	Ian Jackson <ian.jackson@eu.citrix.com>
 M:	Stefano Stabellini <stefano.stabellini@eu.citrix.com>
 M:	Ian Campbell <ian.campbell@citrix.com>
+M:	Wei Liu <wei.liu2@citrix.com>
 S:	Supported
 F:	tools/