[media] V4L2: add a common V4L2 subdevice platform data type

This struct shall be used by subdevice drivers to pass per-subdevice data,
e.g. power supplies, to generic V4L2 methods, at the same time allowing
optional host-specific extensions via the host_priv pointer. To avoid
having to pass two pointers to those methods, add a pointer to this new
struct to struct v4l2_subdev.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>