MAINTAINERS: Add myself as maintainer for Xilinx DRM drivers

Add myself as a co-maintainer for Xilinx DRM drivers to help Laurent.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240327-xilinx-maintainer-v1-1-c5fdc115f448@ideasonboard.com

MAINTAINERS

@@ -7380,6 +7380,7 @@ F:	drivers/gpu/drm/xen/
 
 DRM DRIVERS FOR XILINX
 M:	Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+M:	Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
 L:	dri-devel@lists.freedesktop.org
 S:	Maintained
 T:	git https://gitlab.freedesktop.org/drm/misc/kernel.git