media: v4l2-fwnode: Include v4l2_fwnode_bus_type

V4L2 fwnode bus types are enumerated in v4l2-fwnode.c, meaning they aren't
available to the rest of the kernel. Move the enum to the corresponding
header so that I can use the label to refer to those values.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>