[media] fsl_viu: add VIDIOC_OVERLAY ioctl

Currently the driver enables overlay when running
VIDIOC_S_FMT ioctl with fmt type V4L2_BUF_TYPE_VIDEO_OVERLAY.
Actually, this is wrong. Add proper VIDIOC_OVERLAY support
instead of using VIDIOC_S_FMT for overlay enable.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>