MAINTAINERS: Add maintainer for OMAP DRM driver

Add Tomi Valkeinen as omapdrm maintainer.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>