ARM: multi_v7_defconfig: Enable Rockchip display support

Enable options needed for HDMI out on rockchip: DRM driver, Rockchip
DesignWare HDMI glue and the rockchip IOMMU (dependency of the DRM
driver).

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>