[media] uvcvideo: Add debugfs support

Create a debugfs entry per UVC stream. This will be used to export
stream statistics.

Signed-off-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[mchehab@redhat.com: add incude <linux/module.h> to avoid compilation breakage]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>