[media] soc_camera: Support VIDIOC_EXPBUF ioctl

This patch allows for exporting a dmabuf descriptor from soc_camera drivers.

Signed-off-by: Kazunori Kobayashi <kkobayas@igel.co.jp>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>