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