MAINTAINERS: Add Freescale DCU DRM driver maintainer

Add Alison and myself as maintainers of the Freescale DCU DRM driver.

Signed-off-by: Alison Wang <b18965@freescale.com>
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Jianwei Wang <jianwei.wang.chn@gmail.com>