drm/shmobile: Add run-time dependencies

The shmobile DRM driver is only useful on SuperH and shmobile unless
build testing. I am dropping the SuperH dependencies though because
the driver doesn't even build there, so in practice it is an arm-only
driver for now.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>