Add a new flag that indicates that this control is a dynamically sized
array. Also document this flag.

Currently dynamically sized arrays are limited to one dimensional arrays,
but that might change in the future if there is a need for it.

The initial use-case of dynamic arrays are stateless codecs. A frame
can be divided in many slices, so you want to provide an array containing
slice information for each slice. Typically the number of slices is small,
but the standard allow for hundreds or thousands of slices. Dynamic arrays
are a good solution since sizing the array for the worst case would waste
substantial amounts of memory.
Acked-by: NNicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: NBenjamin Gaignard <benjamin.gaignard@collabora.com>
Tested-by: NJernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: NLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@kernel.org>