media: uapi: v4l: Don't expose generic metadata formats to userspace

The generic metadata pixel formats (V4L2_META_FMT_GENERIC_*) are meant
to be used in conjunction with device-specific media bus codes. Those
codes are work in progress and not available in the upstream kernel yet.
To make sure the generic metadata pixel formats won't be used by
userspace until we have the full infrastructure in place, keep their
definition private to the kernel for now.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>