video: Armada XP: Move driver to DM_VIDEO

This patch moves the Armada XP video / LCD driver to DM_VIDEO. With this
move, the legacy interface board_video_init() is removed from the
theadorable board code (only user of this video driver). The support
via DT will be added in a separate patch.

This patch also enables DM_VIDEO for the theadorable board, as this is
needed to not break git bisect'ability.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>