MAINTAINERS: add Keyang Xie

@xiekeyang has been involved and actively interested get the overall
quality and testing improved for the image-spec.

I'm for enabling this work to continue now as we have the v1.0.0 out and
the work for the certification working group will rely on this continued
focus.

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

MAINTAINERS

@@ -5,4 +5,5 @@ John Starks <[email protected]> (@jstarks)
 Jonathan Boulle <[email protected]> (@jonboulle)
 Stephen Day <[email protected]> (@stevvooe)
 Vincent Batts <[email protected]> (@vbatts)
-Aleksa Sarai <[email protected]> (@cyphar)
+Aleksa Sarai <[email protected]> (@cyphar)
+Keyang Xie <[email protected]> (@xiekeyang)