media: v4l2-subdev: document controls need _FL_HAS_DEVNODE

Control events can be subscribed and received by the user. Therefore
drivers that support controls must expose the
V4L2_SUBDEV_FL_HAS_EVENTS flag.

[As discussed in https://lkml.org/lkml/2018/11/27/637

]

Reported-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>