media: v4l2-ctrls: add support for dynamically allocated arrays.

Implement support for dynamically allocated arrays.

Most of the changes concern keeping track of the number of elements
of the array and the number of elements allocated for the array and
reallocating memory if needed.

Acked-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Tested-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>